I think some sort of floatation is a good idea. I know in my strip-kayak I hardly ever even look into the fore and aft compartments. They leak a little, hatch, fittings... and drain out of the same holes.
Putting some kind of floatation in there shouldn't hurt, and in a serious bull breach could be a life saver.

I think float bags in skin boats and wood and glass boats that have no bulkheads is smart. But as durable as a good float bag is, whatever might cut, rip, breech a hull could also breech the bag.

Two ideas.
1. Empty soda bottles. These things are very tough and almost free. They can take about 150 psi. A mesh bag of these wouldn't fill every crevice, but could fill enough. This is probably usable only in a wood kayak like mine where you'd just like some set-n-forget insurance in the holds.

2. The solid material with the smallest density is areo-gel, probably too expensive to use in a kayak. Close to aerogel and a lot cheaper is the green foam blocks used by florists. This stuff doesn't sponge up water, could be placed into a dry bag and crammed into the compartments (in a skin boat you could hammer down any bulges) --then if you get a rip (hull breach) even if it rips into the bag of green foam blocks--they'd still keep you afloat. Voids would be at a minimum (and anyone who's ever experienced what a couple of inches of water in the cockpit can do to stability would understand how helpful this could be in an emergency.)