I have only been up there once (last year) and fishing time was limited as I was travelling with my family. I believe Weipa/mapoon is an excellent area to explore but we only stopped there for one night and didn't wet a line.
Further north, we made base camp at Loyalty Beach which is a stones throw from Seisa. Tinnies can be launched on the beach. Livies are plentiful off the wharf at Seisa if you can throw a cast net. My favourite fishing spot was in the Jacky Jacky river system. There is a ramp about 20 minutes from Loyalty. It is an extensive system so take a gps and track your course if you head that way. The fishing was awesome but be very aware of the salty handbags - they are everywhere even if you don't see them.
As far as timing goes, we were there sometime in August. The hotter the better to switch the barra on.