I have played with such an idea myself...
the solution to stop the swishing is,
on the inside of the tank, we have a lid maybe glued to some foam this will rise and fall with the amount of water....this lid will be fixed on a runner either side of the tank, which means it can only rise with the amount of water, the runners will be made in such a way that they can only work together...i.e when the boat is level the water can be put into the tank, when the boat is not level the runners wont work, ( prevents sloshing about )
in theory it works in my head......
........not got round to practical installation and trials yet though