Technically there is nothing majorly wrong with them. As a picture though they leave a lot up to the viewer to wonder what is going on. The first does the best in portraying a story: nice pool and house on an even nicer beach with beautiful water in behind. The others just seem to be something you saw and thought you would take a picture. It would have been neat if you could have shown why there was no swimming (trash, boats, animals), what does that sign mean, where is it at, why did you take a picture of it? And why did you take a picture of a barge? Did you think the light made it look appealing, the water it was in, the foreground, background? They're also too centered. Remembering your rule of thirds may have helped paint the rest of the picture and made them more visually stimulating.
A good start though. Keep shooting and the D90 is awesome by the way!!