A fair number of GPS units are waterproofed these days, to an international standard called IPX7. This means they are designed to withstand temporary immersion. For example, an accidental dunk or splash in the lake. Aquapac cases are tested to the higher standard IPX8 which means they would protect your GPS even if for some reason it had to be submerged continuously.
In actual fact the Aquapac rating is IP68. The '6' indicates that as well as being 100% waterproof the case is also 100% airtight and dustproof. Being airtight, the Aquapac will make your GPS buoyant. So if you drop it over the side, instead of it being lost to Davy Jones's Locker it'll stay floating safely on the surface, and you'll be able to retrieve it safely.
If you use any electronics in a marine environment, under continuous exposure to salt air, experience shows that even 'waterproof' equipment will eventually succumb to corrosion. Unless, that is, you invest in an Aquapac.
The patented Aquaclip® seals the case - with a simple twist of a couple of levers. It's been tested to the toughest international waterproofing standards.