Thing is, dehydrating him would mean " touching " him. If you just touch him, you're dead.
Correct me if I'm wrong, but say that he stretched his hand by turning it to sand, if you threw a liquid at that hand, would it turn normal ?
Or let's say for example Crocodile turns into sand or partially, someone throw a liquid at him, he will turn to his normal self right ?
In the case of Mihawk, he has long range attacks, and also, even in close range, it's his sword that will make direct contact, not Mihawk himself.
About Shiliu, well we don't know if they fought to begin with (unless I forgot), but it has probably happened, and taking into consideration that Magellan is in a closed place, which grants him quite the advantage, it is normal for him to win if there was ever a fight I guess.