I would like to point out that if you're looking for Volume that an existing Infantry Bay can carry, it's not going to fill the compartment by weight. An Infantry compartment is big enough that Infantry can comfortably embark, get seated, and check their equipment, then disembark quickly, over the matter of seconds.
There's a lot of open air in that compartment not physically occupied by the soldiers and their kit.
So, the same tonnage of water won't fill the compartment completely. There will be breathing room. But, to fill the compartment completely, you'll probably go over tonnage, and start slowing the vehicle down.