I'm with Bob here,
I take a syringe, voltage checker, paper to dry the boat, a small oiling syringe filled up as all my shafts are oil lubricated. The boat, or boats ,get checked prior to leaving home. My theory is, if it breaks, the lake isn't the place to start fiddling and fixing as I am club fisted and clumsy, and will end up making a bigger mess of it, so no tools means no temptation

Voltage checker as I use lipos on pretty much all of my models, and usually have separate power supplies for the receivers, so its nice to know what my voltages are, and the paper and syringe so I don't transport a wet boat as the water might stain the boat, that's me!