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?