I stuck mine down the back of my PFD

Don't loose your umbrella!

I store my umbrella on the deck of my kayak under the rigging for quick deployment.

If you decide to take an umbrella next time you go, make sure you get a nice golf style umbrella. If you don't get a sturdy one, and you try to sail with it, the wind will just flip it out and break it. A big nice golf umbrella works brilliantly as a sail. I also suggest buying a clear one so you can see where you're going. We had a few near collisions with duck blinds due to umbrella blockage.

More sea kayaking tips from Ali:
Clothes for warm weather sea kayaking