I used to wear contacts all the time. I used to go out in the surf then and it didn't take long before I opened my eyes underwater and lost one .It cost me $70 ( which was a fair bit back in the 90's). After that I got disposables at least then it was only $10 a pop. As others have said you should be able to train yourself to keep your eyes shut.
I found using contacts great other than that.
Since then I had a problem with an allergy to my horse which had a side effect that stopped me using contacts.
In flat water I wear prescription sunnies.Just make sure you drill a hole in the end of the arm to tie some fishing line through which ties to a glass tie.I only attach one end of the tie to the glasses the other gets tied to my wetsuit leash. A Gath helmet also helps keep things in place. Rain X helps avoid vision problems. Good in that they are polaroid so I can see the shallows and sharks better..