Make several copies and lock the bottom image so you have a block to crop the image back to.
Resize the canvas so 50 to 100 pixels of space surround on all sides.
Place a copy of the central image on both the right side and the left side.
Deform the central image to square up the horizontal band of water. I used the distort tool as opposed to simple skewing. Once you square up the image, replace the left image with a copy of the center image.
There will now be "too much" sky above due to the skewing. You will eliminate that later.
Use a blurring or smudging tool on the left and right images to create a blend effect over the central image and erase to make building edges line up.
Flatten the three images together and finish blending and smudging where they meet.
Repeat with the top and bottom edges using copies of the flattened, three-image montage.
Locate the locked original copy under the changes and crop the document to that original.
You will now have a nearly seamless repeat.
Repeat process as needed with new image to get blends at edges right.
To create slipping, I tiled the image on a larger space, flattened it, and cut out the middle row.
I then pasted two copies of the middle, joining their ends together and placed them to offset the design.
There was a little roughness at the point where the buildings met the water. I created a long 50 pixel high rectangle to cover the width of the page and applied the image as a pattern which I could shift up and down in the rectangle to get a good yellow/blue event horizon. I feathered the edges to create a blend effect and smooth the transition between the houses and water, and the sky and water.
Below is the final version.