Boquete Outdoor Adventures website has the best info. You could PM the guy who runs it . I think he is Millerotis on this forum. There is a lot of good boating up there. Seems like there was an outfitter in David that i read about somewhere.
A German guy was starting a kayaking service around Cocle [Santa Clara ?] several years ago,may not still exist, try googling XS Memories.
Renaissance Adventure Guides does trips on the Mamoni ,south [ really east] of the Canal Zone. They may be able to help you find other boaters,boats, or beta on rivers closer to Panama City.
There have been some 1st d's done around Bocas del Toro, some Rio Changuinola. Seems like the east central parts still have stuff to explore