Eric -
It depends how much spray you'll be exposed to (like, will you be standing in surf, or will you be in a zodiac with some splashing and spray?).
For general waist-packs, the best things I've used is a Mountainsmith Tour lumbar pack (http://www.mountainsmith.com/products.asp?productId=28&categoryId=4&subCategoryId=3&subCategory2Id=0) which is incredibly comfortable and supports the weight very well. They even make a version that's from recycled plastics, so it's kind to the environment. It's also the perfect size for a Domke insert (http://www.bhphotovideo.com/c/product/16065-REG/Domke_720_230_FA_230_Insert.html) which allows it to perfectly hold lenses. It'll hold a 70-200 just fine (it's snug with the hood, but I have a rubber hood that works well). I've carried a 24-70, 70-200, 16-35 and extenders in it. It's my go-to bag for shooting sports. It's got interior and exterior pockets as well. I've sprayed mine down with waterproofing spray a few times, and it's done well. I've shot with it in moderate rain, and used it all summer on zodiacs in Alaska. Granted, it's not a dry-bag, but it does a good job.
Another option would be a waist-belt from ThinkTank or Lowepro. The ThinkTank waist belt I have has rain covers that pull out of the different parts on it. You can create a custom bag that will be pretty splash-resistant.
The other option would be to talk to the folks at Sagebrush. I've met them up there in Kake, AK and they're good folks, and have the ability to do custom bags. You could give them specs on someting and design it to attach to a waistbelt system like those from ThinkTank or Lowepro and have what you're really looking for.
Brian