There is alot of questions here but I have inserted some of my opinions into your text, I think that if you are going this route I would stick with gelcoat. To change the color on the boat in gel-coat is the same process as painting it except you have to sand out the "orange peel" and polish after it is applied, but then when future repairs come up it can probably be sanded and polished if its scratches and if it is big damage just grind ,glass, and apply gelcoat saving the primer and paint blending.

just my 2 cent good luck.

dave g

: I just bought a boat and it is all gray's and ivory. I'd rather
: have high visibility colors for the boats I use in the ocean.

: There are some spots that need gelcoat repair. Patching small
: gelcoat sections isn't a big deal, but re-gel coating a full
: hull (and deck) would be another story...

: I'm not that familiar with paint. Is applying it really as simple
: as it sounds?
Yes and no, if you have done it before or have a working knowledge of the products then yes if not it could be challenging.

The boat has lots of gelcoat cracks - small ones -
: do these need to be filled before painting?
Yes, if you do not repair them then they will telegraph through the paint. Paint should be looked at as a color finish not as a way of hiding stuff, paint hides imperfections really poorly.

I've read some
: paints actually add strength, but is the main purpose just UV
: protection? How does this change the maintenance of the boat?
NO comment on paints adding strength, but as for the maintainance of the boat paint will get scratches through the color rather easily and to repair these you will have to paint and blend that area. The gelcoat goes on alot thicker and is harder and in the future minor scratches can be sanded out and re polished.

: There is still gelcoat under the paint. If the boat takes a hit
: and needs a repair does this mean all future repairs will
: require re-gelcoating then painting or can I just match the
: gelcoat to the paint in the future for small repairs?
No the repair would be to grind out the damage, re-apply fiberglass in that area, sand and fair smooth, some finish putty, spray an appropriate primer and re paint the area. On a kayak the blend area for paint depending on the damage could be rather large.