how big? I have a 10gallon tank I removed from my 25... overall dimensions are 18.5x16x8. it was designed and built as a freshwater tank. its black ABS plastic.
personally I see no need or reason for a freshwater tank on a small boat. a water system takes up precious storage room and whatever size tank you install below decks will be so small it will have to refilled very often... and you wont be able to monitor the water usage as well and you will run out. what I meant to say is that you ABSOLUTELY WILL run out.
so you will either need to get to a water hose to refill, or carry the water to the boat in gallon jugs. then discard the jugs. .. and in addition, stow a couple of extra full gallon jugs so when you run out, you at least have something to get by on til you can refill.
if you are going to carry the water to the boat in gallon jugs to fill your tank, why not just distribute them for stowing below decks where they fit the best... then exchange the empty jug as it gets used with a full one in the locker so you save them for refilling.
unless you are installing at least a 25gallon tank(the bigger the tank, the more feasible it becomes), its just not worth it.
its a lot of trouble to install and set up just to say you have running water for a five minutes.... Oops! you just ran out... the 5 minutes of running water was really only four minutes, cuz the pump runs at 2.5 gallons per minute......
but really, you wouldnt leave the tap wide open for that long, but it shows how little water it is when presurized and quickly it can be used up, whereas the two of us can go almost 12 days on 10gallons of water in the jugs, but we could not make 5 days using the tank.... and we didnt drink the water out of the tank either.
using gallon jugs really does save a lot of hassle on a smaller boat....