Every trip I make to CC, I always - and I do mean EVERY SINGLE TIME - bring back seafood with me. I first go to the Fisheries Depot. Office in Belize City and pay for a permit to "export" seafood...I have still to be asked for one, but I do just cuz I believe it's better to be safe than sorry.
The permit then allows me to "export" up to 20 lbs. of seafood, which I can mix up between fish, conch, lobster, and a couple of other seafood items. I always do fish; and whether I do conch and/or lobster depends on which is in season when I go down.
The most important thing is that unless you buy directly from the Co-op in the city, it is advisable that you take any lobster out of the tail and have only the meat.
I generally freeze the seafood for at least one night, in small bags that I can then wrap and duct-tape in newspapers. I put 'em all in a small carry-on and off I go to the US. I have NEVER been even close to have any of my seafood confiscated from me.
While others may be able to give you their opinions, this is mine...I am batting 1.000 when it comes to bringing seafood - lobster, conch, or fish back from CC to the US.