While a kayak will "float" on its own some sort of floatation is a good idea - and may even be a legal requirement. You want to use something light and buoyant that doesn't absorb water.

I have made a couple of 'yaks where I used empty plastic pop bottles as floatation. I prefer the small ones, they pack better. Just squeeze a bit of the air out (to allow for expansion with temperature changes) and glue the top in place with RTV (silicone rubber sealant). A couple of dabs of RTV holds them in place so they don't rattle around. I normally hide them behind a bulkhead in the bow and stern.

I have started using those foam "pool toys" for floatation. Inexpensive, 4 foot long, 3 inches in diameter, they can be easily attached along the sheer where they are not in the way.

Hank