Generally 99% of mammals can swim. Take off the flotation that inhibits movement and let them play in the shallow end. Get them to race from one end of the shallow end to the other, being sure that the water is chest deep. They will begin to swim without realizing it and then you have swimming children.
Babies swim without much training, it is a natural movement. Dog paddling around is what they should do on instinct.
I never remember learning to swim. I just swam from a very very early age.
Also you can try using a floating ball to let them chase it around. A pool with a shallow and deep end would be ideal, sooner or later the toy will get out farther than one of them can stand.
Wolfy(I live on a pennisula, you gotta swim unless you go due north! LOL)