without going into the science, a PL filter increases contrast in the sky, reduces glare and reflections on glass and metal (not metallic paint though) e.g. you could remove the reflection from the glass.
your composition is good (not sure on the skip photos personally) but again; i'd say they are underexposed. for example the second photos, your camera has seen the mass of white in the scene from the slipway and beach, and increased the shutter speed to produce what it thinks is a "correct" exposure, the result of this is loads of missing detail on the car, e.g. where are the tyres, arches, front grill etc
on the sixth photo this actually works well to create a nice high key photo, throw a fill in flash to show the grill / front and you've a spot on photo.
in contrast the photos that don't have an abundance of white, show much more detail e.g. the photo with the skip.
My advice would be to take a look at the manual and look for the exposure compensation setting, this allows you to tell the camera that the scene is more white or black e.g if you're photographing a wedding the bride wears white, this makes the scene more white than the assumed 50% grey, so you dial in a compensation; if you're taking photos at night, then you can dial in a negative to prevent the camera over exposing the photo.
An alternative is to switch to spot metering or to shoot the images for HDR.
hope that helps!