The standalone GPS is now basically obsolete. Most canyoneers use their cell phone or possibly an older cell phone that doesn't matter if it gets ruined.
Whether you use a standalone GPS or an app on your phone, it's important to know where you are and where you are going. I highly recommend downloading offline satteline and/or topographic imagery for the entire area you will be visiting before you leave home. Don't count on cell service in your canyoneering location.
A lot of beta sites including dyeclan.com provide GPS files that you can download and import into your GPS or phone. Make sure to download these tracks before you leave home!