Knowing just the length, it's impossible to say what you should use the boat for. If it is relatively stable then it is probably an older cruising design and well suited to lakes, flat rivers etc. If it is tippy it's probably a racing design. This can also be a cruising boat if you learn to handle it, or just a very good workout boat.

The foam blocks mentioned by another poster are there for stiffness as well as flotation. Usually they run almost the full length, except for the cockpit. If they don't stay in place well on their own, a few glued in blocks will stabilize them. Just don't glue in anything that will hinder an emergency exit from the boat.

Regarding repair, patch cracks with fibreglass as explained elsewhere. Depending on how aesthetic you want to get, you can be very careful about covering the patch tightly with plastic wrap and squeegeeing out excess resin, sanding and fairing etc. Or you can just let the scars show. I'm not too sure about the recommendation on Bondo though. That's OK as a filler for cosmetic purposes, but for structural repairs I'd want a couple of layers of glass cloth in a suitable resin (I use epoxy because it's easy to work with and I always have some around.).

If you want to make it look good, or at least better, sand and fair it out to your heart's delight and then put on a coat or two of paint. I've used marine paint because I didn't feel like scrimping a to save a couple of bucks after multiple hours of work to make an old wreck look presentable.

Finally, be a bit careful about putting an old, patched hull in stressful situations with potentially nasty consequences. Things like surf (can get pretty exciting on even moderate lakes), large waves etc. Just common sense.

The most important thing of all though is to have fun with it, whether your fun is in the fixing of it or, as it should be, paddling.
. Isn't this kayak too big (15')???? Shouldn't this be used for cruising around
: the lake or on a canoe trip?