Is your fuse setup the same as mine? Two blocks of fuses with your horn, bilge, blower and lights on the same block on the left (if you are looking at them facing the back of the boat)?
I have the same issue. I believe it is because everything hooked to this fuse block is ignition controlled. The postive rail will have 12+ volts but nothing will work because the ground side loops through something in the lanyard or key switch and it has a short.
Honestly this is only a theory or educated guess on my part. I still have to track down the exact problem. If you find anything please let me know.
In the meantime, I stuck the lights, bilge and blower on the other fuse block (needed to give up the 12v cigarette lighters to make room). The essentials are working and I keep an air horn nandy to replace the (never used) horn.
I would like to get it right, though. I'll keep you posted if I come up with anything.