This month's "Sailing World" has an article about rebuilding a really old Star monohull, and replacing the ancient flotation with Ping pong balls contained in a bag(s) in areas where flotation can fit (bow & stern apparently). I thought this was a very elegant solution for the following points.
1. typical inflatable or air-filled bags can puncture, and unknowingly, no longer be reliable flotation. Whereas ping pong balls are extremely tough and hard to puncture. the noodles are also good for this, they're tough and immune to puncture.
2. small size would allow being put through small openings, and yet fill larger spaces, and odd shaped spaces. Noodles work with our hull shapes pretty well, also.
3. still allows water to drain under/through them.
4. Weight is pretty light for ping pong balls, although I have not calculated weight to volume ratio for ping pong balls vs noodles, etc. Ping pong balls would not ever absorb or hold any moisture, whereas the foam noodles do soak up some moisture.

Downside - if pingpong balls are not held in place somehow, they might rattle around, making noises that may be irritating. Noodles might be quieter.
Cost - I don't know about costs of pingpong balls in bulk. Retail prices would be prohibitive.