Great Question – and similar to one which used to puzzle me as a child. Why did the sea appear muddy from a distance (we holidayed in Lancashire, England!) but the water was always reasonably clear in a bucket.
The answer has several parts.
First of all, water is not perfectly transparent – it is blue! I only understood this a couple of years ago and wrote about it on my blog, where I showed how you may convince yourself that this is true. So large volumes of very clear water through which sunlight can penetrate will appear blue.
Secondly, water reflects light at its surface – about 4% of the light. Since sunlight shining on the surface is usually brighter than light sources under the water – the sea reflects the colour of the sky.
Finally, particles suspended in the water can scatter light in all directions. If the water is otherwise clear, this can enhance the water’s natural blue colour. If the particles are very fine – like silt or mud – then water appears dark.
Adding these effects together, the colour of the sea depends upon the clarity of the water, the colour of the sky, and the angle from which you are viewing the sea.