This is the C-111 canal.  It has quite a history.   Solid rocket boosters were manufactured upstream and barged up to Cape Kennedy during the Space Race era.   This is an abandoned drawbridge that needed to be opened when the rockets were being shipped north.   This canal was dredged to be over 40' deep so they could get the barges in and out.   I think this was the assembly area:

http://terraserver.homeadvisor.msn.com/image.asp?S=11&T=1&X=1360&Y=7012&Z=17&W=2

At least it seems to be connected by canals and doesn't seem to have much other reason to exist.  As a Florida Native, I had no idea that work was being done for NASA down here.

Aerial Photography of the bridge and spillway