I wasn't aware that just water would clean up spilled brake fluid. If it were me, I'd wash it well with a strong detergent, anyway, to get rid of any dirt or oil that is there, as well. Then, after drying it thoroughly, apply any of the usual products: POR15, Eastwood rust encapsulator are two I know of. Also, I use a lot of rust-converting primers, available various places. Eastwood is one. If you use the rust converting primer, the surface should also get a coat of paint; Rustoleum is fine. That should stop any rust progression.
Also, take a look at the passenger side. If the driver side is rusting, the passenger side also probably is.
Finally, maybe before doing all this, fix the master cylinder. Leaking brake fluid is a mess, and you really shouldn't drive a car with a faulty brake MC.