Hi Rock
Welcome to the TSF!
Trust that the major manufacturers of emulsion have good formulas for making "water-resistant stencils".
Chromaline, Murakami, Ulano, Autotype, Kiwo, etc......
The two-part WR versions are more durable to the acrylic components found in most waterbased and discharge inks.
The formula is only half the answer, though.
Water-resistant emulsions need to be processed completely to gain the best longevity in the stencil.
As you stated, there are hardnening agents that will increase the durability even more,
and even impart short run resistance to some lower-resistance emulsion types.
Always remember, the more water-resistant you make your screen, the more resistant is also becomes to recycling.
Proper reclaiming steps and a pressure sprayer can make that easier.
Happy trails!