Your only option is to use the sail to make a solar still. If you dig a shallow pit at the waters edge, it will fill with sea water. Put a pole in the middle of the pit. Tent material over the pit. Ensure that you fold the bottom edge inwards to make a gutter.
In theory the sun will evaporate the sea water. Which will separate the salt in it from the water. The evaporated water will condense on the inside of the sail and run down in to the gutter. Collect and consume.

Note: The amounts of water will be quite small. You will need several of them. You will also need to remove them before the tide comes in. Shelter in the cave during the heat of the day. Fish for food using improvised hooks and line, Attempt to construct and use a fishing spear. Do not use your knife as a spear point! Fish during the evening and at dawn. Those are the most active times on most reefs. Fish contain fresh water in their eyeballs and in their swim bladder (I think).