word choice - Right use of prepositions in, on, under in relation to water

The dictionaries do not provide good enough answers for the right use of these prepositions in the following context:

1. The man is IN the water.


2. The man is under the water.


3. The man is below the water.

Besides, which of one of these it is correct and why?

• A: The boat is ON the water.


• B: The boat is IN the water.