Blob,
My DS couldn't really swim or ride a bike until he was 11, but he wanted to do both of those things very much, so we kept at it. I believe that the horseback riding primed him to be able to make the leap in both of those. Don't get discouraged, even when it seems slow going. For our sanity and his, we really tried to keep the focus on having fun, and disguised "therapy" as games where possible. Straddling floats and having to keep upright while shooting water pistols at each other in the pool develops core stability, balance, and hand-eye coordination, but I can't say that my boy ever complained about doing that kind of therapy, even when it was really hard.
Individual sports played for fun have been much more successful here, for the all the reasons mich covered above.
I'll also risk losing my "Good Mommy" credentials here by being saying unabashedly that video games have been a great asset in developing visual processing skills and fine motor control for my son.
