Ditto on the Thermos. I always try to bring one along with sweetened tea & half and half on any trip there's a chance of a hypothermic swimmer. Even when you don't need it for an emergency its great to have on float out or at the takeout. Don't think I ever had to dump it out back at home.
I've also brought along what I call the "Armageddon bag" which is a drybag with everything you'd want if you climbed onto someone else's boat and your own gear wasn't anywhere around - fleece top & pants, basic splash / wind gear, wool socks & hat, gloves, summer weight bag, plastic sheet, gorp, backpacker stove & pot with a can of soup and some hot chocolate mix or tea, etc. You get the idea.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse