Having visited more than a dozen ocean monuments in my world, it appears every one generated identically. This generation includes a two story tall empty room, and no sponge rooms. Seed is 380666697, world was created shortly after the release of 1.16, monument exploration has been in various subsequent 1.16 releases. Game is being played with local multiplayer on Nintendo Switch. 

Some of the identical structures:

X: -8,504 Z: -696

X: -8,088 Z: -280

X: -6,808 Z: -1,256

X: -5,304 Z: -920