Steps to Get Aquaman in Red Dead Redemption 2
Firstly get Blackberry
Then get Stringy Meat From Skinning Fox
You can get stringy meat from raccoon as well.
Or Get Stringy Meat From Coyote
After that Go To Pier Near Water In Saint Denis
Then go into the water by riding a horse.
Change Into A Steerable Boat
Press Triangle Button To Command Boat
Hold X Button To Go Forward on the boat
Then Steer Small Boat Next To the Big Boat
Then press triangle to stop steering, once you get close to the big boat.
If you're late then press square.
Press X and Square To Run & Climb
Now explore the ship.
Press L1 + R1 + Select Bottom + R2 to Craft
Press R1 To Bait
Now, don't craft any other, just wait there.
After some time the ship will start sinking.
Now, don't move and sink with the ship.
You've to wait until the ship hits the sea floor.
Now Hold O Button To Exit Craft Mode
Now walk around the ship.
Now you've to to dropdown next to the life boat.
Now, you've become aquaman on the sea floor.
Now Try Lasso On Sea Horses
Try Water Silenced Gun (Water Gun)
Lamp will not shine in the water.
Now try to eat under water.
Now try to see things underwater.
Mark A Mexico Spot On Map
Now Go Toward Mexico
Don't Bump Into Sunk Ship
Continue On To Mexico
You Will Eventually Float Up
Go to Craft Mode To Prevent Drowning
Exit Craft Mode and Continue
If you don't use craft mode then you'll drown.