I'm very rusty, but thinking back on my experience both diving and working as a student divemaster and assistant instructor
the two biggest oh crap moments I can think of were
1) folks with lack of buoyancy control making unintended fast and uncontrolled ascents. I'd dive overweighted just so I had a fighting chance of holding folks down
2) I remember one dive on one of the wrecks down in SE Florida, probably in the 100-130ft deep range. It was a pleasure dive and I was buddied with an instructor. We spotted a lady diving maybe 30ft or so above us with her tank dangling completely loose from her BC. We approached and re-attached for her. Wasn't really an oh-crap thing but as I recall she was nervous.
For your mask concerns, I echo others. I'd suggest getting yourself in a pool or shallow low stress area some day with good visibility. Make yourself negative if you have to just so you stay in control and not worry about drifting off or up....and just do mask clearing drills over and over.
Work yourself up to fully flooding it and just practice taking a few breaths while it's flooded. eyes open
Eventually try taking the mask off (eyes open) then replacing it and clearing it.
Then try taking it off, wrap it around your wrist, open your eyes and go for a short swim before replacing it and clearing.
and yes you can do it in ocean water
I've never lost a mask and don't recall anyone on any dives I've been on loosing one either...but there are quite a few memories of folks with flooding masks and fogging masks.
An upside to getting comfortable with mask clearing....I've many times had a dirty mask and it would be fogging up on my during a dive. I'd just let a we bit of water in, and when the lens would flood I'd tip my head down so the water would slosh across the lens to wash the fog away. No big deal