My son wore a mask at POP last year. For the most part, he won't go under water without it. I'd be curious to see what he'd do if he had to take it off.
Have you just tried goggles? They are way more comfortable than a mask.
For him, it's all about water up the nose. He's my uber sensitive kid -- softness of clothes, tags, noise, etc.
Nose plug. My son is like that too. He has a nose plug for the pool.
Too pinchy. If he couldn't wear a mask, he won't go underwater. He's not my water rat anyway, that's my daughter. I think he'd be content to not even get his hair wet if he couldn't wear a mask. He won't pitch a fit, he'll adapt. He keeps us on our toes.