Bob3
Contributor
Let me guess, your shop doesn't carry any Thinsulate, right?... says fleece is the only undergarment that is any good for diving.

There are several types of Thinsulate, 3-M even makes some for soundproofing & insulating engine compartments.
Boot grade is the preferred flavor for dive undies, don't draw parallels with regular sports wear, the 2 are different critters.
The major advantage of Thinsulate is that it retains 80% of its insulative properties when wet.
One of my favorites for undies are the military surplus arctic weight polypro. I slap some of those over a thin set of polypro & am good for down into the 50s.
Uncle Sam also has some nice Polartec, comes in bibs & jacket. You can get a brand new set of those for under $50.