i cant see any way that you can guarantee that your teeth will 100% protected on or off the water . what you gotta do is look at things realisitcally. if you are going to be paddeling grade 4 then maybe it would be worth the investment, if you are going to be running the boyne, the liffey, the annamoe then a full face helmet and expensive dental insurance is probably a tid bit excessive.
also just because they cost you 21G to ge them doesnt mean that if you do knock one out it will cost the same to replace it. i would imagine that the 21G included the cost of extraction sx, xrays, mouldings and many other things that you wont have to under go to get a replacment.