I have a particular encounter always brewing in my mind where a party of adventurers are going to a sunken temple, only the main way of getting there is to hop across pillars that are only just above the waterline. And of course, the lake the temple's sunken into is the home of a green dragon. So while not necessarily underwater, it's a very 'Jaws'-esque sort of encounter that will punish just about anyone, since no one is prepared to fight underwater.