Yes, but how is that all that complex by modern standards? Dealing with known weight, simply pump the right number of gallons. If your ship can change weight

Yes, but how is that all that complex by modern standards? Dealing with known weight, simply pump the right number of gallons. If your ship can change weight rapidly you might need some fairly big pumps. (I'm thinking of a warship--VLS equipped ships can throw an awful lot of weight into the sky awfully quickly. But on the flip side you can dispense with a whole bunch of stuff meant to deal with the missile exhaust--your VLS tubes go all the way through the ship, when the missile lights it simply blows a panel open on the bottom of the cell and the exhaust slams into the ocean. And it's a safety advantage to be able to jettison a missile without having to launch it or rig a crane.)