Thanks for your comments.
I find that shooting a perfectly straight horizon like the sea makes it
very tricky to get it flat - I had my camera on continuous shooting
mode and took strings of shots with slight variations in angle so that at least one of them would be close to horizontal.
I think I went with the portrait orientation in the first two shots to get more of the close wave and sand detail into the shot, but I don't know why I didn't try a landscape orientation for the more zoomed out third shot. Looking at them now, I agree that the extra space in the sky of the third shot emphasises the feeling of isolation.