1. Either more sky or more of the fort. Try to have your horizon top third or bottom third of the photo.
2. Seems that your colors aren't bright enough. Look at the flag in the first shot and compare the colors.
I really like 3 and 4. Try and keep you horizon level. It looks like the water will run out the right side of the photo.
On 4. I would have cropped a bit more of the right, the black line on the bottom right catches my eye.
Please take this with a grain of salt as I'm still new and have a TON to learn myself. Did you do any post processing or are these straight out of the camera? Where were these posted at? Thanks for posting these.