Is it ok to join a swimming class if i have hemorrhoid / an anal fissure?

Go for it!: Swimming will not harm you, nor will it harm anybody in the pool with you. Swimming is great exercise and will do you well. Focus on drinking more water and eating natural fiber to help treat the fissure/hemorrhoid.

Hemorrhoid: Yes, this is fine. There is no risk to the other participants or to yourself.

OK: won't make it worse

Likely fine: I don't see a problem with swimming as long as you dry off very well after swimming and don't sit in a wet bathing suit for a long time after getting out.