The coins weigh 40 lbs all together, the instrument a few pounds, and the book 10 lbs. The others – crucifix, mask, potions – are negligible. Easily divvied up and fit into Etienne's canoe...
Yet upon checking the previous chamber, there is no sign of the seaweed monsters that had been feeding on the worm carcasses. When you reach the canoe, you find your watery landing point mired in clumps of kelp which the incoming tide has pushed into the sea cave. It should be possible to row through, albeit tough going, but it makes seeing anything in the water near impossible.
The kelp strands have also started to grow up the oar-locks of the canoe, and will need to be cleared from there before rowing out.