Try playing with visual settings and see what you can get away with.

The two bottom settings, water quality and water type, appear to have an impact on the way the units look when submerged. With those settings higher than lowest there appears, at least to me, to be a greater contrast between submerged and emerged units. Perhaps others who are willing to spend more time on this can suggest, if any, what other settings help with submerged units.

That being said, if you move the mouse over any area and hold space, you can see the height of land. If it's negative, it's under water or lava. Around -20 is where units start to become submerged and require sonar to see as well as underwater weapons to shoot at (archer's blast, torpedoes...). Units that can walk in deep water but cannot shoot will not be able to defend themselves starting -21 if memory serves, or somehwere near that value.

Above -20, units are typically visible and terrain is considered shallow enough for most units to traverse, althrough at lower speed than dry land.

Zoomed out, at a glance, you can tell shallow water from deep water where the color is lighter. The center of this map forms a lightning bolt/zigzag across the two center islands. The immediate shade around the islands as well as the lighning bolt are shallow, as indicated by the light tone. The two rocks southwest and northeast also have light tone around them, as do the starting areas southwest and northeast. If you spacebar over those, they are all above -20. The bigger oval that makes up most of the map and is a darker shade is at greater depth than -20, forcing units to submerge if they can't float.