some good questions theere mark, your boat should be ok for the coomera, i did that place for years in a similar set up, the broadwater might be another story, you might struggle a bit with the big boats on weekends but weekdays you would be fine.there is heaps of fish in the coomera and coombabah creek is awesome.
what are you fishing with at the moment plastics or bait??
there is heaps of good whiting bream and flathead in those areas, buying yourself a bait pump would be a good idea, as there is heaps of places to get nippers around there just look for the sand banks with holes and start pumping, here is a good video.
the best advice i would give you is to target a specific fish and work out from there how to catch them, do a search on the forum and you tube is a good resource for info aswell.
good luck