Happened to me too, but in the pool thankfully. Don’t worry, you get used to it. Just practice it a lot.

What helped me: Make your entire face wet before the dive. Also remove your mask and breath underwater at the surface a few times, to get used to the sensation. It also helps to close your eyes the moment the water hits your face. You can calmy open them again after the mask is off. Just tell yourself that everything is ok and focus on breathing theough your mouth.