I would not worry so much about hte water itself as the mold that it can leave behind.
Don't use wood or porous materials... I'd even avoid concrete unless you paint it with Kilz oil-based formula... otherwise you're making a home for mold.
You may have to wash your garage floor with bleach regularly.
Mold is a tough enemy to destroy.
If you can get those no-tools-required commercial-style restaurant racks to put your plants on I'd recommend that, or empty milk crates upside-down... anything that you can take out and hose off once in a while.
This is good practice of any grow op but an absolute necessity where excess moisture is involved.
Have you considered installing a French drain and sump, or putting gutters on the garage to send rainwater elsewhere (hint! rain barrel! unlimited supply of water from the sky!)?
stinkyattic Reviewed by stinkyattic on . Growing in a wet room? :jointsmile: A friend of mine is a few days from moving plants from his vegging area into his newly anointed flowering area, a free-standing "garage" where a bit of water acculuates on the floor when it rains. It's a 12 plant in 12 DIY 5 gallon bubble-bucket w/ drip rings and air pumps-type hydro setup. Local laws are such that he's allowed 12 plants, so he wants 12 big, high-yielding plants. He's got a 1000w Hortilux HPS and a 400W MH, with the intent or running both at the same time Rating: 5