Are kayaks or canoes available to rent at the park?

Yes

Yes. You can rent kayak at SSP

Juniper has canoe rental, last time I was there. Salt run park by Salt Springs rents several type of boats, I believe.

Oklawaha outpost and juniper springs run. 2 great canoe trips day rental

At some locations they have canoes. I didn't see any kayaks. Some of the locations have an office so try and call ahead.