I have dived San Andres on several occasions. The island was not a dive destination for me so I didn’t have a large "stake" in it being good. I believe the waters are over fished, my opinion, because the fish population is a little thin. I might suggest you start on San Andres and do your "real" diving on nearby Providencia...also Colombian. Daily commuter flights are available to Providencia aboard baggage weight limited turbo prop aircraft. I have some photos of Providencia in my profile, "mouthbreather." Providencia is a much smaller, less developed island that has the third largest barrier reef in the world. The diving is world class there. It is reef and wreck dives. No real drift or wall dives for either Providencia or San Andres. San Andres is a tourism mecca for Colombians. Kind of like their own version of Cancun. Providencia is very layed back. Locals speak english on Providencia.....historical story behind that. Spanish only on San Andres. Hands down, I recommend Providencia over San Andres for diving. See below for my recommended contacts for dive shops and travel assistance:
Travel/Dive assistance with Providencia:
Tad, diveoldprovidence.com
Dive Shop/leader Providencia: Felipe Cabeza, felipediving.com
San Andres dive shops: Blue Life dive shop, Educardo, bluelife.com located in the Sunrise Hotel or San Andres divers, Alvaro, sanandresdiverscaribe.com.
Enjoy,
Lew