We can only really survive in a specific and small range of mixed gases.
If we were exploring deep space like they do in the movies, we create conditions we can survive in by making what are basically bubbles that represent our environment/atmosphere, it would be possible to come across a planet supporting intelligent life that only lived in water because for instance, there was no dry land so evolution had developed to those conditions. If they had dexterity, they could make spacecraft and space suits that were filled with water in the same way we do with air.
Yup, a jellyfish with a power drill isn't going to happen anytime soon.