It is certainly a challenge to keep a large animal with such a thick shell afloat. However that would just explain the immense size of the shells. Their shape is just extremely weird and sometimes I’d even expect it to be detrimental to their ability to navigate the open water. If they were planktonic it would not be as problematic I guess but I still don’t see the functional advantage. Maybe mimicry? But of what?

They look a lot like the calcareous shells of some polychaetes but they have a sedentary lifestyle attached to rock or other substrates which is not what we’d expect for Ammonites.

Maybe it’s a puzzle that will remain unsolved.