We have sailed to St Thomas approx 25 times but have never done deep sea fishing there nor ever talked to amother cruiser that has.
I don't think this would be the best port to do a deep sea fishing trip.(sorry)
as cookie said you really don't hear too much about deep sea fishing on St. Thomas, i don't know how good or bad it might be. I would also be very leary of booking an independant tour unless you got some really good recommendations,if something happens and you are late getting back to the pier you could be waving goodbye to your ship.