Some phobias are irrational but that's not part of the meaning of the word

Depends, some pictures I see that get posted on places like that look perfectly fine to swim across to me. Like it's just the sea in a calm day and you can't see the bottom. Actually the same happens at the beach where I live but that is because the water is so cloudy, slopes off steeply though, especially at high tide. So usually can't stand up except right at the shore line.