I really like this question. Because we all know that ocean water when collected say in a bottle in not blue but clear!
So the actual reason that the sea appears blue is that light from the sun is an entire spectrum of colours (that can be separated when you use a prism).
When white light enters the water most of the spectrum separated and is absorbed i.e. the red, orange etc. However blue is not as strongly absorbed and this is what is then seen as the blue colour over sea water.