Do airlines generally prohibit wearing a neoprene suit underneath one's regular clothes?
It could be useful in the event of a plane crash in the middle of the ocean, as a neoprene suit could keep one warm in cold water.
Well, no. There is no prohibition on wearing neoprene.
However, there is no conceivable benefit to wearing neoprene as a protective measure in case of ditching. A neoprene suit would have such a narrow surviveability window that you'd be much more likely to injure yourself donning and removing it.
Basically, it would only help after a successful controlled ditch in temperate climates where rescue is rescue is between 1-4 hours away and all rafts have failed.
Some suggest not wearing any artificial fabrics because if there is a fire or intense heat, your clothing will melt to your skin.
Given that the majority of accidents happen either at take off or landing, you’re trading one scenario for a conceivably worst one.
If you’re paranoid about an accident happening, sit at the very back, wear leather or wool, keep your shoes on, and have a portable smoke mask handy. Or take a Xanax and sleep through it all.