Earth curvature is a "must" if you want to talk about a "simulation" in the literal sense of the world. In the commercial-game-subsim sense of the word it is interesting to include because:
a. it is linked to fuel limitations (so you want to go from A to B via the shortest route) and
b. your targets (convoys) are moving in "shortest-route" type of lanes
Shiping lanes across an ocean would almost cetainly be plotted as curves on mercator type maps (even "S" shaped ones).
Manual targeting is considered the epitomy of sub-siming (which is really great of course), while getting in a good firing position and shadowing are considered "handy" skils also. Why not include good ocean-going navigation in the list too.
