Can you refinish a plastic kayak

Depends on how much you don't want to drill holes in your Kayak...

The camera presumably will accept a standard threaded mount of which there is a multitude of different types available.

If you prefer to use the suction mount it will need a really smooth surface to remain secure. The best method of achieving that would be to bond suitably sized pieces of shiny plastic laminate to whatever flat areas of kayak are available.

If you want a horizontal mount at the top of the peak or curve of the upper deck just forward of the cockpit coaming, an adaptor will be needed. A piece of fine hardwood could be shaped to provide a close fit suitable for bonding or screwing to the kayak upper deck which will in turn present a flat surface for some plastic laminate.

you might want to repost this in the boat section

paint and clear coat

I would say the best thing to do would remove the screws/bolts/hardware
plug the hole with whatever you took out. and buy a new seat with a higher back.
you may need to add those u shaped deck cleats but thats not a big deal

There is really no easy way short of sanding lightly and repainting. (I am told that "tremclad" paint intended for exterior use on metal in tough situations is good for this kind of thing.)

Wax (car wax or wax for fibreglass boats--which is often available in colour) will work for the short term.

I do not know what you mean by a catamaran-style kayak.

I am assuming that it is a plastic kayak,. like most SOTs. Reshaping plastic boats can be difficult. If it was my boat I would try:

1. If bottom flat, lay on flat floor like garage or deck. If not flat, then work on lawn.
   
2. Let the sun (and maybe a bit of extra heat, but not much) warm the bottom so it is a bit pliable.
   
3. Apply force to put the bottom back into shape. If not a flat bottom, then some extra care is required. This is usually done with internal blocks of wood or foam to hold the hull in its proper shape. In more extreme cases you can inflate an air bag in there to maintain pressure over a larger area. (BE VERY CAREFUL if you use an inner tube--it can burst your boat!)
4. Once it is back to about the shape you want, leave it in that position for up to a few days, but at least long enough to cool down.
   
5. With any luck, that will reverse most of the dents. If you don't get it all the first time, you might want to try again. In fact, I suppose it might be better to work on it twice rather than do too much in one go. I really am not sure on that part.
6. With these poly boats, you may never get them back to original shape, but they will almost always accept some re-bending to get back to a more useable shape.

I used the big stack for my river kayaks and it did fine. But my fiberglass sea kayaks are so much heavier that I did not take the chance. For 1 of my kayaks I spent a lot of money to get the special sea kayak rack and it worked great. For my wife's kayak I saved some money and bought some simple v-shaped foam rubber rack attachments for $20. The foam attachments are so much easier, I don't even use the Yakima attachemnts any more. My suggestion - save your money and go with the cheap foam attachments.

get a lens refinishing kit on ebay or at parts store

what is the canoe made of... it may not look pretty but fiberglass will work.... you can sand and paint it later as well