Imagine having unlimited bottom time. Imagine never having to watch a dropping pressure gauge indicate the dwindling number of breaths left in your tank.

The promise of a continuous, unlimited air supply is one of the reasons why I have come to the little town of Port Hadlock on the northeast end of Washington?s Olympic Peninsula. Here, I hope to dive some gear that lets you get as much bottom time as you want.

Under the guidance of former Navy and commercial diver Ron Ault, I am about to submerge in chilly water with a commercial-style surface-supply diving rig, complete with Kirby Morgan band mask, about 50 pounds of weight and 300 feet of airhose. No more of that pansy, color-coordinated, soar-like-a-fish-over-the-reef scuba stuff for me. I have finally got my hands on the gear that real, working divers use. Or, as Ron succinctly delineates the two disciplines, ''Real divers don't wear fins. You scuba guys are swimmers''

With that issue settled, I turn my attention to Ron as he explains the arcane machinations of surface-supply diving and the Kirby Morgan 28 band mask. There are several versions of this mainstay commercial diver's helmet; the one I am using has a fiberglass and stainless-steel front plate but places my head in a neoprene ''helmet'' rather than a hard shell. The term ''band mask'' comes from the stainless-steel clamp encircling the faceplate that holds the at-least-seven-mil-thick hood to the mask.

Inside the helmet, a neoprene gasket surrounds my face and a breathing cup seals my nose, mouth and chin. Equalization is accomplished by manipulating a knob that rotates a nose dam located in the nose/mouth cup. The reinforced glass faceplate rests about three inches farther out, and the frame also holds the communications unit and a check valve that allows one-way air flow from the main air supply hose or the bail-out bottle.

Ron illustrates the need for the check valve by relating a tale about diving a helmet without one. When the hose accidentally became unseated from the helmet, the sudden suction collapsed his lungs and pulled his tongue out of his mouth and into the air-supply port. ''The air pressure was trying to equalize,'' he notes matter-of-factly. ''Hurt like hell, and it was stupid, diving the helmet like that.'' Ron has a lot of stories like that, and over the course of the three days I hear several that leave me slack-jawed in response.

Next, Ron demonstrates how to control the regulator's breathing sensitivity, then explains the purpose of a valve that vents air to a series of holes spaced throughout the mask and can be used as an alternate air supply should the unit's regulator fail. Another valve extends horizontally from the check valve, giving access to the bailout bottle - a scuba-style air tank that serves as a backup to the air-supply line.

In addition to the helmet, my gear consists of my drysuit, a dive computer, knife, light, an old Mk-V leather weight harness with about 50 pounds of lead and my bail-out bottle. Underwater, it actually feels lighter and less cumbersome than a scuba rig.

There are more than a few things about surface-supply diving that recommend it over scuba. One is the dive tender, Scott Batey. You'd think I was Scott's first-born. He hands me my hood and helps me into my weight harness and bail-out harness. He connects my com line, bailout hose and main air supply, and as I sit at the dive station, hands me my helmet.

After I mash my face into the cup, Scott starts the tightening procedure. He zips the helmet, attaches and tightens the thick rubber straps, runs my suit inflator under the harness and hooks it up, then helps me into my three-finger mitts with a deftness I?ve never before encountered. My entire purpose in life during this exercise is to sit, breathe and use my hands to hold my helmeted head erect on my shoulders. While the helmet is not seriously heavy, you find out early on why all the working divers have ear-to-shoulder, linebacker-style necks.

You're Turning Off What? A normal dive team is composed of four people: a diver, tender, standby diver and dive supervisor. We?ve got the requisite crew, and I've gone through the dry-land training, but now I actually have to get into the water. Can you spell ''trepidation?''

Air is flowing and my first job is to get the helmet about a foot below the water. I have a death grip on a hang line. OK, so our boat, the Khia G, is tied up to the dock and I've got maybe 20 feet of water below me, but I'm still not real sure about this. I step down the ladder, ''Uh, you might want to close the dump on your drysuit,'' says boat captain Dan Holmes over the com system as I swing my leg over the transom of the Khia G. My main man/tender, Scott, takes care of that.

I'm in the water. Hey. This isn't so bad!

There's absolutely no breathing resistance; the helmet goes slightly negative, and conversation is the same as if I were on the surface, except for the military ''rogers'' and ''overs.'' Any time someone speaks on the com system there has to be a response, a simple safety measure. Ron's voice comes over the com. ''How you doing, over?'' ''Piece of cake, over.''

Several minutes go by as I acclimate to walking rather than swimming. ''OK, we're going to turn off your air, over.'' ''My and !!! expletive deleted and !!!-ing butt!'' ''Er, roger.''

We go through the bail-out drill, which turns out to be less scary than it sounds. You have 300 feet of hose to draw breath from in all but a catastrophic failure and, depending on depth - of which I haven?t a lot - that could take a while. Water starts to leak into the helmet as the breathing gets more difficult. Time to resort to the auxiliary air tank.

''Out of air, going to bail out, over,'' I say as the airflow wanes. ''Roger. Going to bail out.''

We work the routines we've rehearsed several times during topside drills. Geared up, I find my seven-mil gloves won't grab like my bare hands and I resort to a fist grip to open the valves. We do each exercise until I can literally do it blind. Ron's told me they may cut my air at any time, but after the shallow-water drill, I am a lot more comfortable with that prospect than when it was done to me during the doff-and-don portion of my scuba certification many years ago.

The Real Deal The next morning, we head southeast out of Port Hadlock toward Klas Rocks in the Admiralty Inlet near the Mats-Mats Bay entrance. The weather is gray, blue, sunny, dark, rainy, calm and windy: Basic Northwest dive conditions.

I've suited up in my drysuit already (it was raining when we started out) and the rest of the crew is in the cabin going over tide charts, checking the bottom sounder, referencing navigation charts and essentially scaring the hell out of me without knowing it.

The wind is coming up, but the sea is only running about two or three feet, and I am sitting on the dive deck looking like I am swatting bugs as I do visualizations of the out-of-air drill. Ron looks at me at one point, and I think he actually knows what I?m doing. I get the impression that everyone else thinks I'm losing it.

We anchor between two navigation buoys in a thin bull kelp bed. A harbor seal surfaces to check us out. It's time to get dressed. I am feeling confident enough to let the tender do what the tender is supposed to do.

I step down the ladder and into the water, grab the down line, vent my suit and head south. I briefly forget my ''overs'' as I make my way around the bottom. Ron gets me back on track communication-wise and I start heading deeper.

I am definitely enjoying myself. What do I have to worry about? Compass? The tender tells me which way I am headed, so no problem there. Pressure? The dive supervisor (Ron) is monitoring that. Time, buoyancy, getting back to the boat, running out of air? All irrelevant. I don't have to think about anything except diving.

''Want to give us a depth, over?'' OK. So you have to monitor that, but even depth is limited by hose length and current. I mean, you can only go so far on 300 feet of floating hose, and since they keep the air pressure at roughly 100 psi over your depth somebody?s going to notice a change in breathing resistance.

One disadvantage of surface-supplied diving over scuba is that your mobility goes way down. Of course, the object is to be working, but I am not carrying crow bars, pipe wrenches, cutting torches or mauls and there really is an awful lot to investigate in the land of the Emerald Sea; they do grow things seriously big up here. There are plumose anemones in the most striking white running easily three and four feet high, starfish with arms equaling that and enough colors to do justice to any