Yeah, I like CLC's bungee loop idea, too: very low profile, and easy to crawl over for a wet re-entry.
: 1. What is a good source for both the webbing and the bungee/shock cord?

Camping goods stores or perhaps auto upholstery shops. If you find nothing locally, one possibility for mail order is Hamilton Marine Supply in Maine (www.hamiltonmarine.com to request catalog).

: 2. Is there a difference (in practice, not theory) of nylon vs. polypropelene
: webbing?

Yes, nylon gets stiff with exposure to UV light; I think polypropylene has less tendency to do so. Nylon sinks, and polypropylene floats. Neither of these properties makes any difference for this particular application. The polypro webbing that I've seen is loose and fuzzy. Nylon looks better. Polypro melts at lower temperature and rehardens to a fragile state, making it kind of messy to seal the cut end of a piece of webbing. I think nylon is a better bet, and it's readily available in colors, if that floats your boat.

Good luck, Fred