This is a semi ambitious project - paint a mural.
Paint the top half of the room in a light/sky blue and the bottom half, a darker tone - the color of water, then paint an ocean scene - sailboats, gulls, the sun, clouds, etc. Maybe the shore, with a small fishing village and docks.
It sounds complicated, but it isn't. You can find all types of inspiring photos to help. Take it one step at a time.
I did something similar to the room our grandkids sleep in, when they spend the night. The cafe curtains in the room had a border of wildflowers at the bottom. I bought craft paint, in the same colors as the flowers on the curtains. I continued the border, entirely around the room.
It took me 6 hours to finish it, but it turned out fantastic.
You can do as much or as little as you like. You can do just one wall as the mural, and paint the other walls a solid color.
Take a chance - you don't have to be a trained artist to do this. The good thing with the paint, if you don't like it, you can wipe it off, and start over - BEFORE it starts to dry.