You could think "outside the box" (sorry, that was aweful wasn't it?) and use a decent camera like a Nikonos, which will go through your 40 m deep sump (taking photographs in the process) and when it gets muddy on the far side you can just swill it off in a pool and carry on shooting. In the rush to use digital toys these excellent film cameras seem to have been forgotten. But they're incredibly robust, optically superb and arguably the best tool for the job, especially in the harsh conditions of a cave. The above may or may not help but it's meant to be a serious suggestion.