I would put money on the dog, retrieving a ball with a camera in it throw it enough time and get a good picture

Various ways are possible. Two I can think of off hand are:

The photographer could have a camera on a monopod, selfie stick or something similar so it can be held from above water but near where a toy might be thrown into a pool. This way the person taking the photo doesn’t have to get into the pool.

If the photographer is ok with taking a plunge they could be waiting underwater while someone else throws a toy or gives a command to jump. Having a mask and snorkel can help. :)