I'm pretty sure the reason drywall is commonly used, is that it's cheap, and light colored. OSB is great, but no cheaper than drywall, and if you don't paint it, your garage will be as dark as it is now. OSB is tougher than drywall, for sure. But since your kids aren't likely to be shooting pucks at it any time soon (unless grandkids are around the corner), that may not be a factor for you.
Friend of mine and I put FRP in his garage a couple of years ago. Nice and white, but was not cheap. Sure looks nice though, and you can spray it and wipe it down.
Sluggo, I know a guy who has a garage pretty much exactly like yours. Come over and check it out.
You'll be amazed what a difference a coat of white paint makes.
Regarding the water aspect, I only wash mine out maybe twice a summer (mostly after it dries up in the spring). I haven't found any water damage yet.
If I had a floor drain, I'd wash the crud off the floor periodically throughout the winter, but.. sigh..