Originally posted by vinyblaster:

My problem is not really the depth, but how you can end up with water after digging only a few meters, even when fairly far from the shore.

Well, that's because the water covers the entire map, corner to corner. The ground rises up from there and sticks out, but does not displace the water, like in real life. The water just stays hidden underneath, tides and all. It's kind of a design cheat, but you could think of it as the ground water table.