I think you need to decide what in nature this room represents. There are many elements and parts of nature. It is of course OK to mix, but I feel the mixes somehow need to be natural if representing the outdoors.

If you feel your room represents water, what kind? The sea with shells and sand, or maybe a rainforest river, with a lot of wood and green? Or something else?

Part from your "insane" fish idea, it needent cost a lot of money to fix up. If it is a place for relaxation, try to keep it quite open and decorate along walls instead of in the middle. In the middle you can put yourself if you wish, like suggested yoga, also you can have a small, portable table to put there for tea ceremonies and such, and sit around on the floor. Even though you have a carpet it might be nice to have something soft to sit on to represent the center of the room.

But also arrange so one can sit nearer a wall if that is wanted also.

Color is the easiest way of changing a room, so think hard to find the ideal color. It seems like you already did, use clues from nature what then goes with it.