Ok so here is the report. I fished for about 1 hour one morning in front of our resort in Kahana and caught nothing.
I did however spot some wild free swimming bonefish on our layover in Ohau when we quickly visited Pearl harbor. We went to see the Missouri and while waiting in the queue for the return bus there was a tiny flat next to the ship with a bit of mangroves and in the time I stood there I saw 3 decent bonefish swim across that little flat. Was pretty cool.
Also flying back from Maui the airport flat on Ohau looked great as we landed and I saw two figures out there that looked like they were fishing. I would enjoy a visit there if I was on a family holiday, but from what I saw Hawaii doesn't interest me as a fishing destination, besides everything felt horrendously expensive. We drove a fair bit of Maui in our rental car and I did not think any of the recommended spots looked particularly great for fly fishing, especially wading. Not appealing enough to make an effort anyway.
Maui is a stunning island though and a great family holiday spot. The whale watch tour we did was simply spectacular, the weather stunning and the water warm and clear. Really enjoyed the trip and never really thought much about fishing.