From the Garmin website - on the 2720
"Most GarminŽ GPS units are waterproof in accordance with IEC 529 IPX7. IEC 529 is a European system of test specification standards for classifying the degrees of protection provided by the enclosures of electrical equipment. An IPX7 designation means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes. An IPX8 designation is for continuous underwater use."
This includes the 2720 when you look up its spec. IPX7.
My Garmin has been wet, frozen, dripping with icicles for a week at -10c, dropped, crashed, you name it, it IS waterproof, apart from the battery compartment. So suggest you hard wire it if you expect these conditions.
Mine is an older SPIII, but I doubt if Garmin's standards have dropped any.
I have my doubts whether the card might survive in a metre of water for 30 mins, but the unit should be ok, as for heavy rain...don't worry about it, it'll shrug it off, no worries.