If you are really wanting to 'glue' something to the fiberglass but later remove it without any marks, that is near impossible. Mainly because wherever you mount this thing will not fade in the sun like the rest of the boat, and so you'll see where it was.
To stick things to the boat semi-permanently, use 3M 4200 and 3M 5200 if really don't want it to come off..
Instead of using screws in some areas down under on the inside, I had wondered if there was a glue to use that would not destroy or deteriorate the fiberglass. I had been told or read that be careful what you use.
There are very few products that would do harm the fiberglass and I don't think I have every personally found one that would. I don't think any glue a person could buy retail would have any adverse effects unless what you glue on was somehow torn off, then the better the glue, the more damage there would be.