Convection Heat Transfer Cofficient of Seawater

Clear that you don't understand thermal transfer.

It is not possible to put convection in one coefficient.

Convection if a function of dT, temperature, viscosity, density and thermal expansion of the fluidum. Extra you need to be aware of the roughness, ther size form and orientation of the surface.

A vertical orietated surface has more loss than an horizontal.

This is only the natural convection, you still have to take forced convection into account and the zone in between.

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that is not a straight look up. You are looking for units of W/(m^2*K). The convection coefficient is known as "h" or "h bar" and must be calculated depending on a given situation. In other words it is not a constant. You have to consider boundry layers. There is what is known as local coefficients and average coefficients and both are important. No one will give you a set number for the question you asked. Even though it is a coefficient, it is not a constant.

For plain water against steel, it is in the range of 400W/m2.K