Sam & Dan: Floaty Flatmates

At the end of the game, once you reach the boat, you will ride through a credits area. At the end of this is a water cooler with a big red button; press this and the achievement will pop (and you can fly around)

This requires ownership of one of the donationware DLCs. There are five different tiers you can buy ($1, $2, $5, $10, and $20) but you only need the $1 if you just want the achievement.

An important note: Only one person in your play session needs to own the DLC! So, if the other person in your play session owns the DLC and you don't, that's perfectly fine, as there is a party hat for both people to use. I'm not entirely sure if both people spawn with the party hat in this case, but if only one does, you can still share that one (when I did it, I played with someone who also owned the DLC).

The party hat is on your starting dock. You will need to be in an active play session (the other person is in game with you) for it to appear.