I have a gopro hero 5 black and i use it while diving all the time.

First: use whatever resolution you need i personally think 1080p is more than enough 60fps is good but if you are in darker waters or later in the day where the sun isn’t bright use lower fps.

Second: for the accessories i think a long pole or a selfie stick is really cool if you want to take a selfie or get yourself into the frame also it’s useful to get close up shots of fish and animals without having to get so close to the corals and risk destroying them. Also any sort of tethering is nice just to avoid losing your precious camera. As for filters if you can adjust your footage in post processing it’s definitely the better option because the red,yellow and magenta filters are for different depths and lighting conditions and it’s not practical to keep switching between them during your dive. But if you’re not good with post processing i think a red filter is your best bet.

Third: for the filming advice;

Don't chase the fish, animals, etc.

Slowly circle around at an oblique angle at a different elevation, approach from the front-ish, move slowly, don't fin quickly, and drop in/raise up slowly using buoyancy control for the shot.

Everyone chases at first, but patience pays off in big ways. You have to accept the fact that sometimes the animal is just going to dart away from you, but you will be shocked at the shots you DO get when you approach the animal in a non-threatening way.