- The bottle was floating in the ocean. [part of it is sticking up, part isn't]
The plastic bag was floating on the water.
The bottle was floating on the ocean. [means the surface of the water, no part sticking down]
- The plastic bag was floating in the water. [partially submerged or not all on the surface]
The theme park is on the surface of the water but part of it is submerged. Therefore, floating in the ocean is fine. One could also say on here. It depends on how you look at it in this particular case. With the ocean, it depends on the level of submersion.
The body was found floating in the ocean. [not on: part of it is submerged]
Please note: He was in the house. [inside]. He was on the house. [roof, a surface]
"In the morning" is not the same as a date.
On the morning of July 15 = because of the date.
On July 15, On the afternoon of July 15.
Then, there are times where you get this in Spanish in response to a question: en absoluto and it means: absolutely not. So, en as on is not always so clear.
I have tried to answer your questions specifically, not write a treatise on prepositions. :)