Panic in water

So a couple weeks ago I went swimming with my brother and hes a very strong swimmer and a lifeguard. He wanted to go swimming where there was no lifeguards and I thought it would be OK and relaxing but I didn't anticipate how heavy the water felt.

I'm clostrophobic so I felt like I was being crushed by the water and my head felt like it was sinking. I'm not scared by water and I've done lifeguard training courses so I'm pretty used to swimming in tough places but something about the huge expanse of water caused panic in me.

I started hyperventilating without realising and to this day anytime I think about it or swimming I panic.

How do I deal with this? And has anyone had any similar situations?