IPX7 is a calculated waterproofing rating where the device can withstand being submerged 1 meter in water. If you want to depend on that unit to pinpoint the finish line later in the day when you need it, use the dry bag.

However, unlike the Garmin Legend GPS, I have had good waterproof success with the Foretrex GPS units (the ones with sealed rechargeable batteries - 201?) using two additional techniques to keep the pins from popping. 1) I put some Wind Shear adhesive between the gps and the band to give it some more adhesion. 2) I put a spare wrist strap on my wrist between the gps wrist strap and my hand. The second wrist strap keeps the gps from sliding too far over my hand. I pop the pins when I put my hand down on the trampoline and flex it back. The second strap keeps the GPS high enough that the back of my hand can't get to it.