Often times you are working blind on boats. If you can get a hand on it, consider yourself lucky. You could take a few pictures of it to get a better idea of what you're up against. But, basically, you need to remove the clamp, then wrestle the hose off, then unscrew the nut, then remove the fitting. Hopefully the fitting wasn't put on with something like 3M5200 or your removal job just got harder.
There's also always the possbility that you have to remove something else to get better access or even cut a new access hole.
FYI... it's a through "hull" fitting, not "hole".