As a fellow Gulf shooter I think you will love the 52" commercial. It is designed to be a freeshaft gun with quick and easy lineshaft capabilities. While some of the other have recommended wongs and or riffes (I also have a Wong GR gun) these are not ideal for scuba shooting in the gulf and frankly silence of the trigger mechanism is a silly concern on scuba. I'll get to that though. While Wongs and to a lesser degree riffes can certainly shoot a lineshaft further than any other production gun they take a significant time to reload, especially contrasted with freeshafting or a Biller/Sea Hornet with a speedload kit (200# monofilament and an elastic loop). Most shots taken on scuba are less than 10' from the end of the gun anyway so really all that range is just extra line to deal with. With regards to noise; bubbles make alot of noise, inhaling makes noise, all the metal on metal on your gear makes noise, but most importantly the cavitation from the bands contracting when you fire a gun makes alot of noise. The minimal amount of stealth added by having a "quieter" trigger mech isn't going to cost you any fish in the gulf on scuba. Freediving for white sea bass in Cali I say wear a camo suit and have mirrored lenses in your mask as well as
have a quite trigger mech, Clearwater is not in Cali though.
The biller 54 or the 52" commercial will both be good guns for scuba shooting but I'd change the line on the biller to the speedload kit if you are going to be lineshafting. I am pretty sure Biller has started using the shaft length fired to determine the number on the gun. A 52" commercial on the other hand fires a 60" shaft. Both fire 5/16" diameter shafts, which I like over the skinnier but faster 9/32" favored in riffes. Wong also makes few 5/16" shaft guns.