Cave
A tsunami is the same as a cave. Because, if a wave is big enough, it will curl over on itself, creating a sort of cave within the wave. That's why it's the same as a cave!
Also, imagine a tsunami hitting a cave, becoming the same as the cave (since a cave will shape it).
どう
ほら
None
Best way to remember the reading for this kanji is to think about how the shape of a cave is the shape of a doughnut (どう).
Also note that the 同 part of this kanji gives away the reading as well. 同 and 洞 have the same readings!