[tnli]

I was going to say that it would be a star, but apparently (making an assumption about the mass of the saturn-sized bathtub full of water here) it would still be orders of magnitude from ignition.

An Olympic swimming pool scaled for a Saturn-sized swimmer on the other hand... Ignition!

[saagarjha]

You’d need on the order of hundred times the mass of Saturn to create a red dwarf, so the swimming pool might actually be enough to kick it into being a Sun-sized or even larger star.