I noticed I have a leak where the hydraulic trim line goes into the cylinder. Thought maybe it was loose and started to tighten it but the hose fitting popped out of the cylinder. I have been trying to re-thread the hose fitting back in while the boat is in the water, is this nearly impossible. Could the hose fitting have worked its way loose and stripped the threaded female fitting in the cylinder? Thanks.

I think you better pull the boat out of the water. Even if you get the hose back on there the system is already contaminated with sea water.

Most likely corrosion. The cylinder is aluminum.
You can re-thread with an NPT tap, and try to find NPT male to inverted flair fitting to put on the cylinder.
BTW, take the boat out or at least remove the trim cylinder and keep water out of the cylinder/lines.
Gary