[RPG] Clarifications on combat in water


The rules in the DMG on water in different environments seem to contradict each other.
The section on aquatic terrain claims that creatures at least chest deep in water gain improved cover from enemies on land. It also means, if my understanding is correct, that two chest deep creatures don't get improved cover against each other, but do against creatures on land.
The Marsh Terrain section, on the other hand, says something similar (that water can be used to gain improved cover), but makes no mention of the enemy being in or out the water. But it adds that creatures with said improved cover take a -10 when attacking an enemy on land.
Are these rules complementary, meaning that in any type of watery terrain improved cover granted by water applies only to attacks from the land, and that attacks from the water always take the -10 penalty against land, or do the rules actually change when in Marsh Terrain?

Also, related to the water rules: why does being underwater grant total cover, making most common attacks simply impossible from land, even for weapons like crossbows?

Best Answer

Because they are different.

Note, the water described in a marsh terrain is a "deep bog". The water described in aquatic terrain is just water. The environments are intended to be different and thus have different rules. The rules are not "complimentary" or additive.

Total cover for being underwater is somewhat of an oversimplification of refraction and the resistance provided by the water. I have hunted carp from the bank with a bow and, at least for me, it is largely blind luck determining where to aim. To be fair, on a sunny day, with clear water, with a fish very close to the surface, you can actually target them by shooting "beneath" them, if that makes any sense, but it is extremely difficult to know how much to correct for the refraction. Hence in the rules, it becomes "total cover". Add some turbidity and any significant distance from the surface or poor lighting and it effectively is total cover.