We divide our time between Bogota Colombia and Springvile, Utah. We recently went to San Andres / Providencia to dive. We have use both Louis Banda of banda dive shop and Sea Garden. Banda knows the area better and geared to more advanced divers.
We saw plenty of Stingrays. Squid. Barrcuda. There are many sunken ships around both islands, a sunken 737 (in pieces but still cool) a freighter, and a great night life.
We have stayed at the Decameron Mar Azul and we liked it but thought the food was repetitive after a very few days. (Its an all inclusive) We have also stayed at the Sun Rise hotel. We hated it. Don't go there. TERRIBLE service.
There is a good night life there. Lots of fun things to see on the island. The blow hole was my fave. The most fun, however, is that at night, around 5 or so you can go out to Johnny Cay and feed the stingrays. This was the most fun. I also recommend renting one of the golf carts to take you around the island (about 3 hours).
San Andres is safe, easy to get to from Bogota and not well known... and hence less tourists. Most of the dive operators speak english.