1. The composition kind of follows the rule of thirds. It could have a little more water instead of sky, but that's not really a problem. You can try cropping a little from the top and the left, just so you see for yourself if it brings out the subjects more. Here you might want to adjust the white balance so the white boats don's seem so blue. But this is a good shot, has some repeating elements (the docked boats), leading line, level horizon. ISO 100 and f13 were the right choices.
2. You had good light here, but this kind of light is also hard to capture, especially because of the contrast - bright house, but shadows beneath it. If you shoot raw you can try to work with this later (using curves). Also, if you have auto WB, the camera will always try to neutralize it, so it will neutralize the orange colours of a sunset. You can try using Daylight or CTE white balance for these. The other thing is, you used the 18-55mm kit lens for this photo. The kit lens has a lot of distortion at the wide end (can make things look like they are reflected in a spoon). And the other problem was mentioned by jford - the camera was tilted a little, which exaggerates this distortion. Your other lens would probably perform better here, but I know that you don't always have the time to change lenses, etc. (especially when the sun is quickly sinking!)
3. Nice composition, good idea there and good use of wide angle. The only problem is, the front part is OoF (out of focus). You can make the DoF (depth of field, the area that is in-focus) wider by dialing a higher F-number. But usually you don't want to go above f14, because that can start degrading the overall quality due to diffraction). Oh and btw, if you have a large white object in the frame, it can trick the camera into thinking that the scene is brighter than it actually is. You can use exposure compensation to fix that.
4. Power plant. Nothing really to say. Its a good shot, but maybe has a little too much empty space in the top right corner. I notice here you could fix some chromatic aberration (the slight glow of cyan and red around the chimney to the left). These are tiny things that usually only photographers consciously notice, but they are things that can be distracting. If you shoot jpeg, Pentax can automatically try to fix that in-camera, but maybe only for Pentax lenses, not sure about Sigma. If you shoot raw, you can use software to fix it later.
Overall, I think you clearly have some good ideas about composition and the general settings for the photo. From here on, its just all about details. Thanks for posting and continue shooting