Heck I use a garden hose and a deep sink to clean my Savage and T-Maxx.
Water is only bad if it gets to the electronics and is allowed to sit on the truck for great lengths of time. You should also try and avoid getting water in the engine, but it happens.
If water gets in places it shouldn't like the electronics and the engine, then you should take care of it right away.
For the electronics, turn them off to avoid shorting them out. Dry them out really well before applying power. You could let them air dry for a while, but you would be better off opening up the cases and getting as much excess water out of them before you do. Electronics can and will rust. When they do rust they cease to function as well as they did before.
For the engine, you could flood the engine with nito fuel or oil to displace the water, but you still may get some rust internal. Your best bet is to break the engine down and dry the internals, applying oil where needed to prevent rust. I usually shake out the water and then run a tank of fuel through the engine. Not the best practice, but it works.
So is water bad for your MT? Only if it is left to sit. Run it, have fun with it, but always take care of it.