People seem to think that because we make technological progress in certain areas that we'll magically make it viable to run a machine in salt water in a few years if we just start such a project. It's not like anyone in the entire history of humanity has ever tried running a machine in a salt water environment before. I'm sure if we just build some properllers or some shit down there we'll start farting out new materials over night.
There's probably some pretty solid reasons why geothermal power isn't used more as well.
If there's money to be made of something someone will do it.