I agree with you ,the painting is not the best choice. Maybe get one with some nautical theme ( waves, beach or sea shells). You can just get a poster and frame it in light color frame. The coffee table you can just sand with a fine grit sandpaper to expose original timber and put some clear warnish to finish like the coffee table in the photo. Cushions is easy just get something like in your sample photo, go to spotlight or Target, if you make mistake it's easy to return. Last thing get rid of clutter that is seen in the background . I think couch is fine ,looks comfy.

The dimensions of the room are quite small so you really don't want to introduce a colour that will make the space feel smaller - I personally would have a brilliant white, as the rays of the sun would filter through the windows, bouncing light off the walls and giving the illusion of a bigger room. I also like your idea of either a very soft grey of blue. You shared that your couch was a gun metal grey - maybe paint the wall in a softer shade of that colour to compliment! All the very best to you and your project!

I searched coastal living here on Houzz and Pintrest. Pintrest had probably more suitable pics. Lovely ideas to choose from.

Oh most importantly I forgot to add, it’s a massive big sand hill I’m building on. 1500sqm lot, which I’m subdividing and selling one lot and developing the other. The back of the block which faces the ocean has a 6-7m steep drop, with an equally steep front access. I have been perusing all the architectural designs and reading lots of info on Houzz - which I absolutely love! I think I’ve even found my architect from your site too.