Maybe use a mix of blues, such as turquoise and navy, along with the white, and the green carpet. These could evoke ocean colors. Not sure if the rug is a true green or an avocado ( which may not work). Some nautical or sealife touches can be used such as round porthole look mirrors, model sailboat, curtains with seashells or fish, etc.

I bought my daughter a beautiful expensive king single at 14 and she used it for a year then decided to use the cheaper pine double I had for visitors. Teenage girls are very tricky.

Very nice. Is it MDF or wood and where did you get it from?

I would definitely get them full size single beds, whichever solution you go for. You could get them each their own loft bed with heaps of storage and a desk or pullout desk underneath each. There are lots of different variants of these and some would even help to make them each feel like they have their own room or at least space. If you would like bunk beds that can still be their own single beds later, you should look into 'Bunkers' as this is their trademark. They are Australian made and all pine as well so no nasty chemicals. Pretty sure they also have the loft bed options and desks that you could put underneath. They do bunks that can go at right angles to each other with storage underneath in the corner as well so you could put the beds in the corner opposite the wardrobe and desk near the wardrobe instead of under the window if you prefer this to their own loft and desk/storage underneath:-)