On the cowl lock, this is what I've done to keep the water out:
cut a 1/8in. donut of silicone rubber tubing and stretch it over the cowl lock on the exterior side (carefully or with needlenose pliers...I broke a few stretching them over the lock).
you're basically makind a small O-ring to go on the outside of the lock.
When you press the locking mechanism, it still locks, but is snug and the pressure keeps a seal because of the silicone O-ring.
So far I haven't had any water in the boat since doing this.

I also use pool noodles and hatch tape. Always tape the boat!

Another great flotation idea I saw on another forum(I think...can't quite remember) was pingpong balls grouped and shrink-wrapped together...light weight & clean/cool looking.