The skinny island looks more like swamp to me as well. I think it may be because there's too many rivers? Maybe try a single river from each of the main mountain portions (top and bottom-left) without any branching?

With the tidal delta, your "swampy" part looks a bit too brown for that. I'd try a darker green, like what you have on the "don't know if it's swamp or jungle" island.

Want to know which island is my favourite? The one at the top left of the island "circle". I keep looking at that one and thinking how much I'd LOVE to see a full-size map for it!! I just love the general shape, and how the mountains curve around that inner portion with all the islands.