[simonh]

Did you read the whole answer, including the bit at the end?

>...So the major reason the oceans look blue, if they do look blue, is indeed that they mirror the sky.

[StavrosK]

I contest that, the sea looks blue in overcast days too.

[marcosdumay]

Hum... The sea color changes a lot depending on the sky condition. In overcast days it's way more likely that it will be green than blue.

(And, of course, after it rains there is a good chance the water will actually be green, but it's a different timing and a different shade of green.)