Well, if you want to mimick real life, then Polynesia should be in the middle ages in 1850.
I always thought badly about food bonuses until I realized that a tile that had plenty of food just allowed your city to work on less food tiles, therefore, you have more citizens available for other, more productive tiles. Or specialist slots. This is not the case for tiny islands.
Currently you don't settle on tiny islands until you can work some specialists, or it will starve for production, no matter how many fishes you have.
Before doing a rework on Polynesia, what's exactly wrong with current uniques?
Trirremes building like work boats is something you use with secondary cities, or rebuilding after an attack, or settling in tiny islands when you are ready.
The toolkit is too focused on maritime things, so it is extremely dependent on map. But playing in a balanced map this should not be an issue.
The moais require settling in the coast, or at least one tile from the coast. With current bonuses, they benefit more from peninsula like formations (moai triangle), while with your proposal they will benefit more from 1 tile wide long islands. It's not that I dislike it. I find it more broadly useful actually, but current bonus is not really bad (just luck dependent on your map).
The extra food on fish is not bad if you have other interesting tiles to work on. This means that, instead of working 2 food 1 production tiles, you can be working on 1 fish 2 production tiles.
Your proposal will make viable to settle on some 1 tile islands surrounded by 4 fishes. That's 12 production doable in Ancient, enough to make a start. But do we really want Polynesia settling on tiny islands in Ancient?
The only problem I see with Polynesia is that it is very luck dependent. You have to find nice empty lands for you to abuse, with tons of fish and sea luxuries and irregular coast lines, or you are screwed.
If you want to remove a bit of that hazard, then I think one of the following can be useful:
a)
+1 yield (production?) to all water (not ocean) tiles, instead of bonuses on resources. This way, even if you have 0 fishes and 0 sea luxuries, you can still work some sea tiles. Even if you happen to start next to an inland sea or a big lake, you can work it. There are some maps (Communitas, I'm looking at you) that are pretty low on fish. This will also make golden ages very valuable to Polynesia (sea tiles with a Lighthouse will yield both gold and production).
b)
Moai base 1 production 1 culture. Moai gets +1 production for every 2 adjacent sea tiles and 1 culture for every adjacent moai, plus tech scalers. This way, if you place a moai on a tiny island, it gets 4 production 1 culture, moais in a straight coast gets 2 production 2 culture, while placing a cluster in a peninsula (quite rare) gives 2 production 4 culture each moai. So it becomes a bit less map dependent in that you benefit both from peninsulas and very coastal exposed tiles.
Now the only irkness is that Polynesia has 0 incentive for working on specialists, at least from her uniques, but we know that coastal cities end up working lots of specialists because their territory is somewhat limited.