For one, you'd have to pay 50 bucks to get it replaced. Second, it wouldn't be a brand new iPad that they would give you. It would be a refurb. So technically if you dumped your working iPad in water right before your apple care plus ended you would of spent 150 bucks for a refurb iPad 3rd gen.
Moral of the story is yes it is worth it in the long run. It also adds resale value to the iPad if you want to sell it before the 4th gen iPad comes out.