What causes Surfers' Rash?
Is it irritation from rubbing on plastic?
Is it some microscopic creatures getting rubbed into the skin?
Or something else?

rash surfing

It is when the board rubs against your skin. You get it when you paddle out into the waves. That is why it is a good idea to wear a shirt and board shorts. Some people wear a wet suit.

It mainly comes from the wax which you use for grip rather than the board itself. When l learned to surf l used a soft top so there was no wax, my skin still got irritated from rubbing up on it so much but it was not nearly as bad as once l was using a board with wax. In the warmer weather you will usually see people wearing a shirt designed for surfing called a Rash Guard. It will keep you going good :D

if its on your belly wear a shirt if its on your legs its the sand plus rubbin on the board

If you have a soft top board the soft top might be soaking up bacteria from the water and also if you dont change your wax regularly it can have the same effect what l would suggest is wearing a rash guard (Roxy and quicksilver is a good brand) or wet suit or maybe just a t shirt. but also mkae sure you scrape of all of the wax on your board and replacing it with new about every three days. Hope this helps!:)