The bug

Unlike other structures, the Y-coordinates at which ocean monuments generate appear to be fixed and do not adjust for the surrounding terrain. This can cause odd results for superflat terrain, and may wreak havoc for customized worlds (although I'm not sure how to check) as it may not account for changed ocean levels.

Since this was fixed for desert temples in 1.18, I'd expect ocean monuments to also be patched, but this does not seem to be the case.

How to reproduce

I've tested this using a modified Water World preset, which has ocean monuments spawn by default. The changes made to the preset are shown as follows. The water has been moved down in all three examples for clarity - note that this also triggers MC-206214.

Test 1: Water World, water layers changed to 9

Test 2: Water World, water layers changed to 9, deepslate layers changed to 32

Test 3: Water World, water layers changed to 9, deepslate layers changed to 96

Expected results

The vertical positions of the ocean monuments would adjust to fit the surrounding terrain.

Actual results

Instead of the position changing, the supports either extend downwards to a comical extent or the ocean monument carves a huge hole into the surrounding terrain.

Further notes

As terrain is variable, it may be desirable to forbid ocean monuments from generating above a certain height since they're meant to be underwater and having them be on the surface wouldn't make sense. This height value could be configurable via data packs such that default worlds would have a reasonable maximum level.