For a pass through window to be very useful, it needs a surface to place items that are being passed through, that can be easily reached from both sides, or you would need to coordinate people inside and out to pass things, which isn't feasible.

I would consider whether it will be used as a pass through, given the door is immediately beside it.

A small breakfast bar that uses the window for a view can also be useful for a pass-through set-down surface. Make sure the window is large enough to give a line of sight from person to person so that it is easy to communicate between the spaces - along the lines of 'Could you grab another beer?' or, 'I forgot the sauce.' I would also make the pass-through window a little wider if you do have a seating area, so it looks big enough to be welcoming, or it will be less likely to be used.

At 2.500 deep, the deck is not large enough for a comfortable outdoor dining area of more than a couple of people. You need 600mm for comfortable seating on each side of a table, plus the table width which will be 900-1200, and with this layout, you also need another 900 mm to allow access past this, to the pass through, the seating and to the steps down, and with the deck being above ground level, space for access beyond the seating on both sides of the table is a good idea. If there is a barrier, you can get away without it because chairs won't fall off the edge, but it does make things tight. That adds up to at least 3000-3500 and preferably more.

You can make things work with a tighter space, with a compromise to fixed seating along the edge, and a narrow table.

Cute house plan for the beach! The things I would change would be to make your toilet/bathroom doors into pocket doors to give a bit more space there and make your hall cupboard into either a set of sliding doors or if it is holding games etc, it could be a nice recess with shelving in a feature colour or wallpapered behind.

For the outside, could you move the steps to the outer edge of the deck so you can create a bar under the pass through window and I like the post above about creating an inside breakfast bar - maybe turn the dining room around to accommodate that. Again, if the doors going to the outside were a slider, this wouldn't encroach so much on your deck space.

For colours, I would introduce fun beachy colours into the bedding, artwork, throws & cushions on the sofas. I would add some vibrant plants in the corner of the living room and maybe some potted herbs in the kitchen and a plant in the bathroom.

Best of luck!