I think I've got it figured out. I'm going to have the fore and aft access hatches like you said, one underneath the seat, and one in the back of the console. They will both be watertight, which can be pretty easily accomplished with weather stripping. I'll also have a side access panel in case the plumbing explodes. The aft access hatch will be the main one in use, because it'll be easier to get bait, rather than having to lift up the seat. But, when the tank is not in use, it can double as a fish hold, and the seat access will provide a larger opening to stuff fish into. It's all comin together...
-Brian