Hey, this is because you have no indirect light built into your light maps. This is one slightly confusing aspect of the default reflection behavior right now. Screen space reflections do not rely on ambient light so they will also ways show up even if you don’t rebuild. But screen space reflections can only reflect what came from somewhere on the screen which is why you get what appears to be vanishing reflections along the edge of the screen.
Outside of those screens space reflections it should blend to the static cubemap reflections.
The quickest way to get some indirect light is to add a skylight and rebuild lighting. Make sure that your skybox geometry is further away from the skylight than the threshold set on the skylight actor. Or just manually specify a cubemap. Then add a sphere cubemap capture actor and set the radius bigger than your water.
You can also use an the levels world properties and specify an environment color and intensity there. Or you could use an ambient cubemap inside of a post process volume.
In terms of making reflections fully dynamic, you can still use a full render to texture reflection it will just be pretty slow. In most cases, the screen space reflection adds enough motion that it makes the reflections feel dynamic enough.