There are two main kinds of waterproof handheld GPS unit : mapping and non-mapping.  A non mapping unit will be fine for easy tasks such as locating your way back to the vehicle following a day hiking.  You’ll need a mapping GPS unit if you’d like to map your trails or plan your walks ahead of time.  A mapping unit allows you to download maps from your PC so that you can have them with you during your travels.  You can not do this with a non-mapping unit.   

When selecting the right water resistant handheld GPS unit you’ll need to give some thought to how it will be used.  The built in antennas on some units are better than others and you need one which will be able to pick up the satellite wherever you travel.  Bear in mind that if you’re in deep brush just about all GPS units will have a problem.  You can always add on an external amplified antenna that may be worn on the top of your hat or backpack if you find that you need one.  When choosing a water-proof GPS unit think about the amount of internal memory the unit has.  This will determine the quantity of detailed maps that it is going to be able to hold.