I've been referring to this article by James Spencer for Larsen. I've never really see him lose a bird in the water, but it can happen from time to time when the birds are wet and low or if there is a chop on the water. My normal method is to simply give two pips and say "other way", which is his signal to run or swim in the other direction.
I'll work more on the "back" command, which I've been told to do with multiple retrieves. That is, place two objects together at the base of an overturned bucket or traffic cone some distance out. Get the first retrieve. As he sits facing you, with his back to the bucket, command "back" and essentially tell him to go back to the bucket for the second bird or object.