Here's a idea that I picked up in a outdoor survival book.
I think it would be a valuable addition in a cold water ditch kit.
I've used it car camping to warm my sleeping bag.

I don't think many ditch kits will include a sleeping bag, so other than clothes on back and whatever you have in kit, your space blanket/bag along with you fire is what going to keep you warm.

Get some leather sheeting from a craft store. I found some stuff that looks like the leather they use on tool pouches. Cut 1 piece about 12" square and the other piece 12" x 3 ft. You can make it any size you want, the ratio between the 2 pieces is the important thing.

Using a hole punch, punch holes on 3 sides of the square and 3 sides of the rectangle piece, to match the square. I used leather bootlaces to lace them together, going around the edges as you go. When you get done, you have a pouch with enough leather left, to wrap around the pouch, giving you a double layer of protection. Punch 2 holes a couple of inches down from the middle of the rectangle piece. Thread a suitable length of leather/nylon cord to be able to tie packet together.

Depending on conditions, you may/may not be able to keep fire going all day/night?  When you can, heat a suitable sized rock until it's sizzling hot. Be careful of river rocks, they can explode from built up steam. Put hot rock into leather pouch, wrap leather flap and tie off.

Inside a space bag, it will keep you warm for hours. I bought a piece of soapstone for my car camping set-up, but I don't think anyone would want to haul rocks into the wilderness  ;D