I do like the plans you are forming. Anything like a mangrove with roots in water would always be nice but those roots might start to penetrate. Sounds like your really thinking well on it. When I built my pond in the fish house i had opportunity to put in roof lights for daylight, ran a waterfall and had lots of tropical house plants on a 3ft wide decking area 1ft above the pond which also provided a dark area for the fish. It was great for a while but the plants were taking out too much oxygen on a night and giving me filter swings. Then the umbrella plant in the waterfall grew massive, cutting out most of the light, and whilst the roots were great for removing nitrate they needed regular pruning so I ended up with a builder's plank in there to get accross, spoiling the effect almost constantly.
then of course was the massive difference in condensation between just running water from a pump return and a waterfall, the walls were just running!
nothing here is meant to put you off, cos I wanna see it in place, just food for thought so you get it right by at least thinking about the mistakes of others.