Atlantic Ocean color(s)?

It will depend a lot on the weather conditions and sea state. The water color is largely a reflection of the sky, wind will also play a factor as the rougher it makes the water, the less "shiny" it is. The water in the Atlantic is clear, just like in the Pacific, Indian or any other large body of sea water. It all depends on the weather.

Unless it's a bright sunny day stay away from blue. I tend to like black.

What part of the Atlantic are you wanting to model? Here in the southern U.S., water colors can change from crystal blue to green depending on how close to shore you are.

Thanks. Probably a grey-green color.

German sub so probably mid Atlantic ? A lot of area to cover. I was thinking a grey-green?

These are relatively calm sea states:

An overcast day in the mid-Atlantic

A sunny day

A hazy day

I've seen the Atlantic about every color in the paint rack from Sky Type S to Oily Black (and some of the Alclads, too :) )

However, I seldom had a chance to appreciate the Atlantic from a model-like perspective (consider just how close that bridge wing, or even a masthead is on a model versus where one's eye normally is).

The suggestion above, to start with a base of black is not bad.  I've used a rather black green (schwartzggrun) for a base coat.  Then used artist's acrylics in black, ultramarine, and titanium blue mixed wet-on-wet into a coat of gel medium over the base coat.  Which looks a wretched mess in practice, but comes out pretty cool every so often.