Well, as an unlicense medical practicioner, I feel inclined to answer this question. External hemorrhoids sticks outward, and is covered with mucous membrane. The soft mucous membrane is prone to get irritated by butt wipe paper. Then they ulcerate, bleed, and hurts. Sometime the bleeding is profuse.
It is best to push them back in (if they are not scarred) after you have a BM, and before you wipe. If you can't, use a sitz bath. They can be installed on the water supply line of your toilet (if you don't mind cold water spurting at your butt). Then you don't have to wipe. If you are regular, and like clock work, by all means, jump in the shower after your BM
Vaseline is a great barrier ointment to get them to heal. Over the counter stuff are either astringent or antiinflammatory, so I would recommend that you focus on prevention. These stuff wouldn't "cure" hemorrhoids.
Problem with diving isn't the nitrogen, barotrauma, etc. Problem with diving is the swimsuit would ride up the crack and irritate the rhoids. I would go butt naked under the wetsuit.
If that doesn't work, just go buck naked. But like other's have said, avoid the cleaner wrasses. If they nibble on that, next thing you know, a herd of reef shark might visit your hinnie.