I recently played Newer Super Mario Brothers and I noticed that they had many custom liquids in the game, how do you make these? One of which was boiling water which did one damage to your character and could be swam in. It behaved like water that will hurt you. Does anyone know how to make these? Would Hex Workshop be possible? Any help would be great thanks!

That's a question for New Super Mario Bros. Wii, not New Super Mario Bros. DS. This forum applies only to NSMBDS, which is the DS release of the game

But, I do have an answer to your question. There is a sprite in the game that looks like bubbles and blows you towards a certain direction underwater, which is used in the original game.
For the boiling water in Newer, they just disabled the collision for the bubbles meaning they will NOT blow Mario towards a certain direction, and put an invisible and non-solid hurt tile throughout.

Basically, it's not a new liquid, it's just normal water with the bubble sprite and invisible hurt tiles