I hate to be that guy, so I won't. But I will lol Darts almost never interact with water features, they aren't buoyant enough to swim and will drown very easily if they fall in the water. even shallow pools & streams aren't really worth it, they just don't spend much time in them & they are a total nightmare to maintain (I removed a stream from one of my old dart vivs less than two weeks after setup, it got nasty and leaked). The best thing to do would to add a 2" thick layer of small & large leaf litter that the darts can safely hid in without the risk of drowning or foot rot.
To the OP, I would suggest a black mangrove lagoon for the build, if at all possible (I know blacks are hard to find). They grow a finer branch structure & smaller leaves, and have really neat roots that grow upwards out of the soil as opposed to downwards. Things like clingfish, sponges and tridacna clams would make for a really neat tank!