I use these sometimes:
Clicky
Basically a waterproof ziploc. Then I stick it in a small zipper bag with a strap and stick it under my wetsuit. Pain in the ass but it works.
If you don't have a push button start like on a Prius, you can just get a regular key made and take that in the water. Leave the other one in the car. I do that with my Sienna van. If you have a new car, the alarm is probably activated when you lock the doors regardless if use the remote to engage the locks or just lock the doors manually.
With my Prius, there is button under the steering wheel that turns off the ability of the key to start the car (unless it is in the key slot) and open the locks. I remove the door lock key and take that in the water and hide the remote key in the car.
One thing good about a Land Rover is the keys are waterproof.