This article has a list of what chemicals to avoid, why, and some good sunscreen choices.
The three main points are below. Note that titanium dioxide falls under physical sunscreen but is still harmful! So saying physical is okay but chemical ones are not is FALSE.
I personally have used Raw Elements and Stream2Sea before and been pretty happy with them.
Avoid sunscreens containing petrolatum, commonly known as mineral oil, which takes years to biodegrade, and are known to be harmful or fatal to aquatic life and waterfowl.
Avoid sunscreens with high content of Titanium Dioxide. This mineral does not biodegrade and is found to react in warm seawater to form hydrogen peroxide which is harmful to all sea life.
Oxybenzone and octinoxate, the two chemicals recently banned in Hawaii and are believed to cause coral bleaching.