You mean that the power that forbids the U.S. to build warships doesn't check up on you to make sure you're not defying the ban?
Even someone who needs instructions printed on the heel to pour fluid out of a boot should be able to spot a big hollow cylindrical space running along the ship's spine. On a good day he might even be able to spot 50T bays.
Hans
If it's labeled on the plans as "aquatic landing flotation/ballast tankage" or "liquid bulk Cargo Hold" and installed with lightly bolted in panels every 6-12m...