Light and a whistle. Multitool as usual

Map of the boat seems like a good idea, as do decent shoes and a credit card / cash.

None of this is too bulky or arduous to carry.

A chemical heat pck might be useful, if only to keep hands functioning in cold / wet conditions. Lightsticks might have value tomark position for extended periods and spare flashlights. Decent gloves might also be useful but we are now starting to stretch what might be carried on body?