In the old days it was easy. Tie an old shoelace around your neck and loop one end of it through your old fashioned car key. Wear that inside your wetsuit or rash guard vest and you could always get back into your car at the end of the surf session.
Then along came intelligent keys with remote control, infra red, batteries inside them and who knows what other magic. Great for the average driver but terrible for a surfer or watersports fan who is definitely going to get them wet.
So what are your options these days?
1) Get a valet key that doesn't have any electronics in it and wear it round your neck. This could be a boot key or a skeleton type key for your driver's side lock
2) Get a small waterproof bag or pouch, put your regular key in it and wear it round your neck. In theory it should work but the slightest failure in the waterproofing and you are locked out of your vehicle so there's always a risk.
3) Get a key box. A key box is like a mini combination safety box that you put your keys in then hook and combi lock it to your vehicles tow hook or door handle. This is the safest option but if you are away from the cars on a long distance all day paddle it would give a thief ample time to go through every combination of the lock and get into it.