In reply to Blue Straggler:
never claimed myself, but have been part of the process for clients making claims. as a trip provider, claims can be checked with you by the insurance company so getting verification from any operator your mate was with (boat, dive company, rental etc) helps.
any photos from the day also help.
i have noticed tho that dropping stuff or simply loosing it can be seen as negligence and therefore not covered. stolen, lost due to accident etc are often ok tho.
true, its somewhat up to the insurance co to disprove it, but they may take the easier path of a simple call to the trip provider, scare them with talk of 'giving fraudulent information' and say no.
in your mates case, not lashing it to a buoy wont look good, but if you can show there was no secure way of doing or whatever you may have a case. be warned tho that may come back on the trip provider (unlikely for a small claim of a camera, but you never know).
it will help if an image shows you tried to clip it in, rather than just a camera with no means of doing so (yes, they bring shit like that up if they want to).
what id do is get in touch with the trip provider to arrange the evidence that 'all due care was taken' and submit the claim with them onside.
ps, even if the 'trip provider' was just a mate with a boat or whatever show there was some sort of agreed relationship so its square with the insurance co if they check up on it. essentially you want it to cost more to investigate than to just pay the claim.