Most modern remote keys are, while moderately resilient to a bit of water potentially at risk of dying a death if given a through dunking. The transponder (the bit that actually let's you start the car) is typically more likely to survive. Still, killing the remote part is still going to be an expensive problem.
If you've got a flip-style key (as opposed to the keyless push-button start Intelligent Access Key) you could get a non-remote key cut and use that - the transponder in a non-remote key is going to be rather difficult to kill by getting it wet.
As @mikes' answer mentions Ford do offer a keypad entry system which you could use to not have to carry the key with you. It basically sticks to the car so can be easily retrofitted and you can then store your key in the van (obviously out of sight - otherwise some enterprising criminal might just decide to smash the window in and drive off!)
I've got the nagging feeling though that this might be something of an X-Y problem - would it not be better to invest in a decent quality dry bag for $10-20 and instead keep the key in that when you're kayaking?