Hi Westie!
Mine has 3 aft through-hulls on each side. The lowest (with a rubber flapper on the outside) is the passive cockpit scupper drain, which makes a 90 degree flat turn. The other 2 output the bilge pump and fishbox macerator pumps. If a big guy stands in the aft corner he may put that scupper far enough underwater so that seawater can flow into the cockpit oozing past the flapper (but it has no direct connection to the fishbox drain). I consider it good practice to stick the garden hose in the drains from the inside to flush em out, since a lot of cockpit debris, leaves etc all gets washed down there, esp at that 90 degree turn, and ensure no debris is holding the flapper open. It's easy enough to replace that scupper assembly from the inside, although it won't do any good if the real issue is the drain is too close to the waterline. If they used 5200, you’re hosed. Some owners plug the inside cockpit scupper with a stopper when having heavy company at the dock or launching like Brent. Not advised when underway. I've never had that issue so far (Triton hull #0001) though I did buy the stoppers. I read that the starboard listing issue was addressed by moving the 30-gallon water tank to the slightly forward port sponson (that’s where mine is).
The fishbox drain is plumbed to a macerator pump on each side, with the output sent up a loop and overboard when the pump is turned on. Any water in the fishbox can't get past the pump until the pump is turned on (there is no fishbox float switch unless the owner installs one, and fish would crush it or clog it). Seawater can’t enter the fishbox macerator pump thru-hull due to the loop being within an inch of being as high as the sides of the cockpit. If seawater is coming in over the side of the boat that way, you have a lot more of a disaster than water in the fishbox.
Cruisers disconnect the fishbox hose/clamp where it enters the macerator pump and let that end lie in the bilge, so that errant rainwater that gets in the fishbox drains into the bilge and gets pumped overboard by the automatic bilge pump/float switch, also through an upper loop (did someone do that on your port fishbox?). That way the fish boxes stay drier (not dry) and our rear anchor, lines, fenders, hoses and other stuff we store in the fishboxes stays drier. If you never, ever put fish in the fishboxes that's a good solution. But you wouldn't want fish scales etc to drain into the bilge.
Despite the gaskets and cockpit scupper drains all TC255 fishboxes seem to suck down rainwater or any other water by capillary action, or perhaps by magic.
Good luck!
John
