Depends on how fancy and finished you want it to be. Can be as small as a5 gallon bucket or as large as a 55 gallon drum depending on needs and size of boat. Can use coolers too, but round is better than rectangular.

You can put an overflow outlet towards the top and an inlet outlet towards the bottom or anywhere.......through a bulge pump on the I let side and either have it suck water in or even have it on the end of a hose and throw the hose with pump overboard to pump water in.

Lots of options for any size boat or budget.