Using a Polarising Filter to Reduce Reflection and Increase Colour Intensity

Bronte coastline - no polarising filter

Bronte coastline - polarising filter set to optimum position.
No Photoshop
Note richer colours in the ocean and darker blue in the sky.
You can also create this look in Photoshop but it takes time...

Polarising filters have three functions, not all of them positive.

Advantages:
- Use them to intensify colour, especially blue skies and green foliage (in fact anything that is reflective will intensify when Polarised because once the reflection is removed/reduced, you see more of the original colour).
- Use a Polariser to reduce reflections from shiny surfaces. 
Note: This only works when the sun is behind you and reasonably high in the sky
- Polariser filters cut two stops of light out of the scene so you can use them to slow the shutter speed down - regardless of whether the colour changes, or not.

Disadvantages:
- Polarisers only really work well in some light, typically bright sunny days when the sun is behind you.
- If the sun is not at the right angle, the colour shift in the sky might appear uneven - darker blue in one part of the sky, for example.
- Never use a Polarising filter in dim light, especially when hand holding - the filter reduces the exposure by two f-stops so you will suffer significantly more from camera shake.
- Never use them indoors - there's no colour benefit and you'll get camera shake!

Ethiopia, near Lalibela.
No Polarising filter

Here's a good example of how a Polarising filter can enhance the greens and yellows in a scene.  All foliage has a hard, shiny surface (to minimise evaporation) so if you reduce/remove that shininess you get to see the real foliage colour, NOT the blue sky reflected in the leaf.
A polarising filter also enhances the yellows and helps to minimise haze.

Straight shot with no Polarising filter.
Rainbow is captured quite nicely

Interestingly though the colours are stronger (because the reflections have been reduced),
the Polarising filter removed the rainbow entirely...

No Pola filter.
Camera set up in the dull shade, pointing towards the light.

With the Polarising filter fully adjusted (rotated) you get slightly stronger colours - but also a slightly 'deader' looking result because the highlights have disappeared. 
In this example, all the (sky) reflections in the rock pools have disappeared.

Timing is everything when making long exposures with moving water.
The longer the exposure, the wispier the water, to the point it might just appear like smoke.
2 secs @ f22, ISO 100, Polarising filter.

 This was the same length exposure but the waves had receded a bit giving me more depth in the rocks. Every shot will be different.
2 secs @ f22, ISO 100