Yeah, the salinity is the deciding factor as far as I'm aware too. Sometimes though, natural features where named before a scientific definition was known to the namers, which is what I meant by "one of those imperfections in naming conventions that make a world feel more authentic". Looking at the map, we can see it's a freshwater lake, but the settlers who first came there might only have heard of the sea as a very large body of water, which the Rainbow Sea is. That's how I explain it to myself anyway, I don't actually know what the thought behind it was