Port Philip bay is often infested with blue blubber jellyfish when the water gets warmer. It can be like riding through a massive field of pumpkins.
If you get a direct hit you crash. I've never damaged any gear, even at kite foiling speeds and lightweight kite foiling gear.
If you're riding slow enough you can slalom through them. If you keep the foil up high you can bounce off the top of them. That's kind of fun. Badoomp, badoomp, badoomp.

Contrary to popular belief the big jellies are pretty solid. You don't cut them in two if you hit them.
They tend to be in patches. If you keep riding out far enough you can get to clearer water.