Not all plants melt once submerged. For some things, like HC for example, you get better results faster if you grow it emmersed - it grows *much* faster and has a chance to fully root in the substrate (HC is tiny, plus it floats, so it can be difficult to get it to stay down in the substrate until it's rooted). Once submerged it continues to thrive.
I have two tanks I started at around the same time 3 months ago. Both were meant to have HC carpets, one I started emmersed and the other submerged. The tank that was started emmersed was fully carpeted within about 6 weeks, and I've had to trim the HC 4 or 4 times now because it's growing so well. The other tank.. the HC has spread out some, but it's not a carpet by any stretch. I planted about the same amount of HC in each tank in the same pattern, so it's not like they were done drastically different - one of them just had the benefit of 3 weeks of air. The ironic part is that they do have different substrates, but the slow growing tank supposedly has the "better" stuff. I'm actually thinking of moving the inhabitants out an draining the slow growing tank to start over with it.