There is no simple answer to the question "What Color is the Sea?" because it depends on a range of elements, such as the weather, the depth of the sea, how much wave action there is, and how rocky or sandy the coast is. The sea can range in color from bright blues to intense greens, silver to gray, foamy white to polluted slick.
The four photos above are all of the same small stretch of coastline, but look how different the color of the sea (and sky) is in each. They clearly show how the weather and time of day can change the color of the sea dramatically.
The top two photos were taken around midday, on a sunny day and on an overcast day. The bottom two photos were taken not longer after sunrise, on a clear day and on slightly cloudy day. (For larger versions of these photos, and several more taken of the same stretch of coastline, see the Seascape Reference Photos for Artists.)
When you're looking at what color the sea is, don't look at only the water. Also look at the sky, and consider the weather conditions. If you're painting on location, changing weather can have a great impact on a scene. It also influences which paint colors you select.