Getting used to deep waters

I (39, F) live in the Caribbean. I started taking adult swim lessons a little over a year ago. I have a strong freestyle, can tread water reasonably, and able to swim in rivers (fresh water) and beaches (salt water).

However...

I am terrified of swimming in water over 6 ft. It's completely mental. I swam even in deep waters but stayed at the most shallow parts and didn't last long.

It's holding me back from my fullest potential. What can I do to mentally overcome this? I almost drowned in a river at 9 years old.