The Deathfish: Key to Immortality
I haven't really talked about fishing as a skill, because frankly I find it pretty boring. Although on the up side, it is one of the few skills that children and teens can work on. (Kids can even fish from swimming pools!)
You can take a fishing class for $400 to start the skill, or you can just start fishing. This is not a skill that you can learn from the basic skill books. (However, there are books you can read to learn how to capture specific fish. Frankly I think these are kind of a waste of money, since your Sim eventually learns how to catch those fish on their own.)
Fishing is not a particularly productive way to make a living. But you can tell your Sim to go fishing, and they will fish until their needs drive them to cancel the action. So it has that going for it!
Out of all the fish you can catch, the Deathfish is the only one you can do anything interesting with. The other fish can be cooked into recipes like sushi, by Sims who have enough cooking skill. Or you can sell them for cash. Or if you have ten of a particular kind of fish, you can click on a pond and stock it. This allows you to catch an infinite number of those fish in the future. You can also use them as fertilizer in your garden. You can stock them in a goldfish bowl and keep them in your home, although you have to keep feeding the fish and cleaning the bowls. And finally, you can send your fish away to be taxidermied so that you can display them on your walls.
It's not that there isn't a lot you can do with fish; there is. It's just that all of those things are boring.
But the Deathfish, ah, that's another story! The Deathfish is a key ingredient in Ambrosia, which resets your Sim's clock back to the beginning of their life stage. (Sadly it will not revert them to the previous stage. An Elder who eats Ambrosia will still be an Elder.)
The Deathfish is also notoriously tricky to catch. First, your Sim needs to have level 7 fishing skill or better. Next, you need to know where and when to catch it. The Deathfish is not available all the time - you can only catch it between 11PM and dawn. When the Deathfish are active, a white mist will form over the water.
Deathfish bait: Angelfish. You can catch Deathfish without using bait, but it definitely improves your chances.
Deathfish Locations
Riverview: Go to the cemetery on the south end of the central island. Go to the tip of the island directly behind the cemetery. Look for the "No Fishing" sign. Deathfish will spawn in the water about 10-20 feet directly behind this sign.
Sunset Valley: Pond in the center of the graveyard.
Buccaneer Bay: Between the rotted pilings from the abandoned dock, at a beach roughly the opposite side of the island from Hogan's Deep Fried Diner.
Bridgeport: Pond in the center of the graveyard.