If you kick/spread your legs while swimming that could really cause a problem, so avoid that. The water istself shouldn't be a problem, even chlorinated water.
I swam and used chlorinated spa water last year and never had a problem.
Have fun!

Ohmyarse - that is soo hilarious - I'm also a senior finishing my bachelor's degree in psychology! How cool! (I think we're the same age as well).
Luckily at my Uni, lectures are recorded so even if I miss one or two, it's no big deal - I can catch up at home while sitting on my luxery bum cushion (or standing up/lying down/doing whatever I want). It sure beats driving there, sitting in a hard chair, etc. But I've also sat on a bus for 12 hours with my fissure, so hopefully you'll be ok
try following lecia's advice (with leaning back a bit /slouching so the pressure is not on your actual fissure, and more so on the back of your bum where there's more soft cushion-y-ness (i know, its not a word)..
take care