Having grown up in Miami and done a lot of diving between Miami and Key Largo, a lot of it from Carysfort Reef south, I know a knife isn't a problem. The real issue is spear gear. I've been inspected multiple times by park police and DHS, and the only issue was spear guns/ pole spears. I've had coolers checked for fish, and once had a close shave with a hogfish in a cooler that was shot outside the park but we dropped in for a fun dive...bad move, but no more than a warning came of it. Basically, the park officer had seen us spearing and followed us when we came in and then pounced us...Actually, we thought we were outside the park by about a hundred meters, but our chart was outdated. I was stopped another time with spear gear on board, but no fish...so no problem.
Where you might have problems is with a stiletto-esque knife for braining fish. There might be some questions about whether you've got a bag of fish stashed somewhere or are poaching lobster traps/ running a ghost trap. Even then, though, I doubt it.
If you're diving off a dive charter and not a private vessel, then you're not going to able to poach anything, even if you wanted to, so you should be fine.