(Unidentified submerged objects.. not that I believe in “objects”)
“71 percent of the world is covered by water.
It should not be surprising that we should also look there!
In my view of course…as an ex nuclear engineer…i have to point out that water makes excellent radiation shielding (its used in American nuclear war ships for that purpose).
If “our hypothetical friends” were either photophobic (like in mythological lore), or radiation sensitive (like in my PSPH),
Or simply did not wish to be tracked due to emitted radiation (radiation is present in most of the best cases),
Then deep in the ocean would be an excellent choice for many reasons..