When you say auto opening, are you talking about a switchblade or an assisted opening knife? A switchblade has a button or some sort of system that will open the blade without you having to touch it, whereas an assisted opening knife (commonly referred to as an AO knife) has a spring that doesn't activate until your have opened the knife a little ways (I think it's normally between 10 and 20 degree's, but I could be wrong). They are both easy systems to use, and I've seen some AO knives that are a little faster than a switchblade.
As for the rusting issue, most stainless steel knives won't have this problem. But if it's going to be in that type of conditions 24/7 then you'd probably be wise to get a rust repellent product. If you like, I can give you the link to some stuff I like to use. It's called Tuff Cloth, and it's designed to leave a thin film of rust inhibitor on the blade to prevent any buildup. It's good stuff, and reasonably priced too.
I'd recommend taking a look at some of the Spyderco knives. While they aren't assisted openers or switchblades, they are tough knives that will handle a lot of work. I own the Endura Stainless Steel model, and it's one heck of a knife. It's pretty light weight, but since it's made completely out of steel it's very strong. And since it's a simple lock-back knife, there isn't a lot of places for rust to build up, and cleaning it would be very simple should it need it. Kershaw makes some good, dependable, and reasonably priced assisted openers. I have the tanto point version of the Blur, and it's a great knife. Not sure if it's exactly suited to your needs, but you might want to check it out.
Fixed blades will always be a little easier to maintain in an environment such as the one your describing, since there's no moving parts or little holes for the salt water to get it. You can get some really nice fixed blades that work with both your size limit and budget. I'd personally recommend looking at a SOG. I know some on here don't care for them, but I really like their knives, and they have some hard working fixed blades that are designed to stand up to abuse. I don't have the links near by right now, but I'll post them up a little later on. They are the company that used to make (and I think still does supply) the US Navy Seals with their standard issue knives. I've seen some of the tests the Seal Knife goes through, and it looks to me like the thing will stand up to just about any abuse you'd care to throw it's way. That particular model might be a little large for you, but they also have smaller versions as well. Like I said, I'll get back to you with some links later on.
