So I realized too late that my Kerfur had driven itself into the lake on its trip back from Tango. And too late to see that it had been there long enough for its battery to die completely. Is it savable? or is it a lost cause without scuba.

I do have a hook and can manage to get down next to it under the water, but I can't seem to figure out how to drag it out. I have even tried hooking it and teleporting to base with rather amusing consequences. (IE: Flung about to my instant death) 

Try to hook down next to it underwater and grab it to drag it back up.

This happened to me as well. Your best option is to try to grab him quickly as you if can. It might take a few tries.

Turn off physics damage, hook him, then teleport to base. Should fix it.