ring up garmin and cry to them is the only option. I busted mine and they fixed it for free! Although i suppose I do have a far better reputation as all-round nice person.
You were lucky. I had two Garmin handheld GPS fail through water, one seawater in the cockpit and the other a heavy rainshower. Both did not meet the standard of 1 metre of water for 30 mins. Garmin did replace both of them but I had to pay for them. I was not impressed although I had previously heard good things about Garmin service.
flippinek they took their time eh? All the while you had no portable GPS! Very VERY dangerous. You aren't going to stand for that are you? Course not. Get a sheet of sandpaper and write them an abrasive letter. Or cardboard if you want to write a stiff letter.