Great idea Ian! I have used a similar setup Razz, not on a swim platform however. I usually a nylon lock nut spun onto the bolt "upside down".
Tighten the bolt, tighten the lock nut to tension the bolt and make it harder for the bolt to back out.
If that setup all loosens and disappears into the depths, well you must have pissed off some higher power...
Cheap enough a setup to keep spares on hand if it really bothers someone enough to want a quick on the water fix...
Kraig - 2010 Outback V