I've worn disposable contacts on all of my dives and have yet to lose one. Seawater is more saline than tears so the lens tends to shrink against the eye and remain on even when it's necessary to open your eyes underwater (I've tested this in salt and fresh water and have only lost the lenses in fresh, so I'm sticking with my theory even if it's BS). I'll usually close my eyes if I have to demonstrate a mask clearing for any reason, though when I had to do a full mask removal and clearing in mid-water at 90' during a tech class, I cheated by squinting as it was otherwise difficult to maintain the same depth when my eyes were closed. Obviously if your mask were completely knocked off your head, you'd need to open your eyes to find it, but worst case scenario is having to abort that dive if you lost both lenses (you might want to put lenses in both eyes when you dive as a form of redundancy). As I've never lost a lens, I don't even bother bringing spares aboard a day boat anymore, but certainly have some packed back at the hotel (or when on a liveaboard) just in case.
J, on the other hand, has a custom prescription mask. She had two, a new one and an older one for redundancy, and managed to lose both on one trip in Bonaire (one has the lens shattered as she handed it to me and I dropped it onto a rock at Karpata, the other one misplaced in the commotion after she broke her ankle at Old Blue), so we got a new one. Unlike disposable lenses, you need to treat your expensive custom prescription mask with kid gloves, preferably bringing an equally expensive spare mask in case the original were lost or broken. J has a bad habit of setting the mask next to her as she doffs her gear and I'm always quick to snag it before it flies off the boat or someone else sits their tank on it. Also, she's practically blind if she doesn't bring her prescription glasses to change into right away. One time she left her glasses back on the Palau Aggressor when we were taking a skiff ride to and from the dive site. Unfortunately that was the day that the crew decided to give us a little boat tour of the Rock Islands and she had to view it through her mask which was getting quite fogged up on the surface.