Hey Elvis,
I'm no Whitsundays expert, but have done a few self guided trips in my trailer boat. Whitsundays Tackle World at the BP on the RHS coming into town, is well worth dropping in to.
Personally, I've found the fishing around the islands fairly tough. Langford Reef just on the edge of the Green zone has held some nice trout. There's GTs around the bottom of Hook is when the tide rips through, but you might struggle to stop them with less than 50lb gear. Cid Harbour is a sweet anchorage as is Nara Inlet.
Can you go to the outer reef in the hire boat?