Big removable white cooler with a plumbing kit you can buy at basspro or cabelas. Its a nice setup for jons and any cartopper really and removable if you have no need for it.
One boat has built-in, in the other a cooler with a few inches of water & a gallon jug of frozen water. Most fish will die, but they're gonna anyway, and they stay nice & cold & fresh.
Mine are built in but I have seen guys use the rubbermaid containers when they put them in the deck, before I had these I always used a cooler. If the fishin aint good atleast your beers cold.