Least regrettable but most unconventional liquid to take a bath in?

Gallium? It's solid at room temperature, but your own body heat will melt it, so you lie down on a solid block of metal and then slowly sink into a melting puddle in the middle of it. It's non-toxic and six times denser than water so you'd be really floaty on it too

Heavy water - like water, just heavier.

Maybe coconut water or aloe vera gel, I imagine it'd feel weird but probably not regrettable.

I guess we've got to beat Cleopatra as our baseline with her bath of asp milk.

Marmite would probably veer into the regrettable category, and I'm saying that as a marmite enjoyer

Water based lube.

It’d have to be a communal bath though, and I’m inviting you all, even Hairy Steve.

Oobleck for the experience. It wouldn't clean you, but you'd have a story. Possibly mild regret if it's in a bathtub that you need to clean or a house who's plumbing you're responsible for afterwards.

Heinz Baked Beans.

Sleepytime tea

Oatmilk, Tea, Cum, Flavored water (no sugar pls)

Alcoholic beverages

white wine (but a dip into red would be interesting)
beer, for the carbonation
champagne, for the bubbles and the decadence of it

Other liquids

sparkling water, a jacuzzi without one
rose water (for those who don't know, it is sickly sweet smelling and very persistent and can be food grade)
oil, like sunflower, olive or any other of the like. After, just scrape you body, like the ancient athletes would do

Elon his blood. 6L is enough.

I was going to say heavy water, but TIL that it's not just chonky water. It can be toxic.

Macaroni & cheese

A mixture of Vaseline and baby oil, just enough to keep it liquified.

Zero sugar energy drinks.

No sugar means its not gonna get sticky, it'd rinse right off.

I imagine a bunch of creams already use the b vitamins you'd get since they love shoving b vitamins in those.

And it'll actually drain when you're done and a quick rinse will get everything normal afterwards.

Vanta Black

I'm gambling that the experience of it would mitigate how much of a pain in the ass the repercussions would be.

Edit: I suppose I ought to have looked before, but this appears to be not the healthiest decision (who'd have though‽). Maybe we'll go with some kind of closest equivalent nontoxic paint?