a local dermatologist actually recommended salt water and sunlight for my friend's pretty serious psoriasis (in addition with the other stuff) so the ocean diving could probably be beneficial I think. Saying him like "travel to Spain or Portugal couple of times a year and swim in the ocean+have a lot of sun" and "taking regular saltwater baths in your bathtub may also help"
If your friend has small wounds or other more serious damage because of the psoriasis then those probably would be needed to be taken care of first so that he is not risking infections. After that, ocean diving could actually help his condition