Jackson isn't very toy motivated in general- he's picky, but he loves fetching these, I think because they are so easy for him to grab. The little tennis balls etc are not easy to grab in the water.
We use the small bumper from Chuckit.
Chuckit! Amphibious Bumper, Small
These are fun too
http://www.chewy.com/dog/west-paw-zo...e-dog/dp/43048