Some sort of spandex shorts like UnderArmor compression shorts work well under baggies.
I usually use Belly Jelly every time i hit the water as well. Belly Jelly comes in a white and pink tube and is marketed to prevent wetsuit rash. I find that it lasts a bit longer then Vaseline, but that could just be my mind justifying buying the slightly more expensive product. In a wetsuit i put a bit around my neck. In trunks later up the inside of my thighs. If im wearing a rash guard I put some on my stomach and nipples. It works like a charm.
And yes, I ONLY use it for surfing..... damn perverts...
Ya hate to spend cash, but I would try the new Hurley Phantoms, the material is much softer for that area. That or make sure your rails are smooth, even if ya have to sandem down a little. Or you can but this product on here for it: http://www.surfinghandbook.com/wetsuit.html
Better to know now before the Hurricane season comes about..