I wouldn't base a whole keep on swiming. I would rotate it in the workout regiment. I've never done it either but this would probably be how id do it.

HW till empty.
10 min on mill
5 min on springpole or flirtpole
HW till cool
5 min in swim tank
HW till cool
5 min in tank or on mil

Increase as dog develops.

Now if you have a dog that will fetch a stick. Instead of adding the swim to the workout regiment I would just every third day or so give him/her a fun workout day and play fetch with a stick in a pond or lake