I have used a Silva UN Marine for a number of years and am quite satisfied
with it. It comes with a mounting bracket which can be bolted onto the boat
with some modifications (it's designed to fit on a flat surface), after which
the compass can be easily plugged into it, or unplugged and stored. You can buy
additional mounting brackets at a very reasonable price if needed.
I seem to trade off between different boats so I purchased a quick mount
which according to the label says it is manufactured by Paddlers Supply
Company, Escondido, California (760) 739-8363. It is designed for the Silva
compass and quickly affixes to the deck with bunjie cords, so now I can easily
move the compass and mount from boat to boat.
All that said, these days more often then not I simply use a small hikers
compass which is tied to the cord on my paddling jacket and rests comfortably
on my sprayskirt. It's always with me so if I get caught in the fog it's not a
problem. I really don't think your average sea kayaker really needs the
sophistication of an expensive marine compass or even a GPS for what we
typically do. But I admit that I like a good gadget as much as the next guy :-)

Scott
So.Cal.