- "There are creatures living underneath the sea
So come along, let's count them, one, two, three
There are whaladons and Quarren
And the coral worms are borin'
And they're all the slaves of Mon Ca-la-ma-ree"
- ―From A Child's Guide to Dac
Dac was the most important water-world ever. More important than Manaan and Kamino combined. It was so important, there were at least three kinds of sentient races there. Everybody loved the Mon Calamari. Everybody hated the squid-looking Quarren. Technically, the whaladons were intelligent beings, too... sentient beings with valuable oil and blubber and great-tasting meat that nobody could legally harvest... but you can find 'em on the black market. Call me later, I'll hook you up, bro.
The Mon Calamari and the Quarren were always either at war with each other or living uncomfortably side by side and planning the next war. Eventually their planet joined the Galactic Republic, even though they still hadn't unified into a planetary government... which was a violation of the Prime Directive, but the Republic didn't have one anyway, so they got in no problem. They were represented in the Senate by a corrupt Quarren asshole named Tikkes.
During the Clone Wars, Dac was attacked by the Confederacy of Independent Systems, but were saved by Kit Fisto and his amazing underwater lightsaber. By the way, don't try this at home, kids. Ordinarily switching on a lightsaber underwater is like throwing a toaster into the bathtub.
Palpatine and the Galactic Empire brutally subjugated Dac, because both the Mon Calamari and the Quarren had a distinct fishy smell to them which reminded Palpatine of his ex-girlfriend, who he hated more than anything else. Unfortunately, this savage treatment made a young Mon Cal named Ackbar very angry... and he began designing traps and eventually became a galactic hero.
Other important things about Dac: when the damn Yuuzhan Vong conquered Coruscant, all the Senators ran away and set up their new headquarters there. Emperor Trioculus went whale-hunting there. The Empire used World Devastators there once. They invented a freaky kind of underwater ballet with antigravity bubbles. Oh, and Darth Krayt conquered it, too... only instead of mistreating everybody, he turned over planetary control to the crappy Quarren, who were only too glad to do the mistreating themselves. Then later Darth Krayt genocided the heck out of the whole planet anyway.