I started with a bumper in a little thicker vegetation. Just started out with doing marked retrieves, them slightly more difficult... once she started searching on her own I added the 'find it' command when she got close. Repeated to make it a little tougher each time.
Hotdogs... I read somewhere they are really good to use to teach your dog to dive under the water. My dog naturally tended to dive for branches on the bottom of the pond to play so much so that her whole body would submerge. So I figured maybe if I can re-inforce it, that's what I started this weekend and I got her reaching down to the 2nd step in the pool with her head completely under water going after the hot dog. Will it translate to diving after cripples who knows, but fun 'training'