In my opinion, those are too many additional opportunities for water to get the better of your boat . A good quality hiking short or pant would have grip areas right on the material. The grip would stay where ever your "fanny" needs to be to manuver the boat. I do not know if you can purchase a liquid latex or rubber anti-slip material that could be sprayed or rolled on to a regular pair of sailing shorts like the anti-slip texture on the bottom of a bathroom rug. Just my thoughts.
I have an old pair of Bodyglove neoprene swim shorts that I use for sailing they give me some padding and do a good job of sticking to the deck. You might also look at this product http://www.kiwigrip.com/index.html It's something I'm looking at useing on my O'day DS2 but might do a good job for you in this case.
If you don't mind the look, take a pair of comfortable shorts or cut-off jeans/old jean shorts. If they are thin and you want more padding, you can sew on some denim patches to the butt cheek areas. (You can get fancy and make pocket patches into which you can slip foam sections as needed.).
Caveat: Not responsible for any possible toxic component to this fix, but I will try it when I get back home and let you know if it works or if I break out with a rash that sends me to some top gun dermatologist.
(I think I will) try this: With the shorts OFF you, smear on (or make a pretty design) some silicone caulking like that used for around tubs and sinks -- the sort that remains rubbery. Take some waxed paper or plastic wrap and put it over the silicone, then press the caulking into the fabric. Remove the plastic or waxed paper and allow the silicone to dry and to cure before wearing the shorts. As I write this, I'm thinking that perhaps the pocket behind the "patches" would be a good idea since you can temporarily insert some waxed paper to prevent the silicone from going past the exterior layer of the shorts (the "patches"). This way your skin wouldn't be in contact with the silicone and less chance there would be any sort of contact dermatitis.
If you find it drying before you get to smear or design enough, alcohol may help (as you do when you're running a bead along a faucet or tub surround).
For the artistic, it comes in colors. Add a few sparkles/glitter from a craft store and you've made your butt a laughing stock. You can be the butt of jokes, that is.