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!)?