Since you will be working extensively around saltwater, I would agree with Mistwalker that stainless would be a good idea to think about.
You might want to take a look at Gravelle Knives, they are customizable and made by a Blade Forums member. I have not had the opportunity to handle one of his knives myself, but I have heard only good things about him and his knives. His field knife has a 5" blade, comes in S35VN or 3V with G10 or Micarta handles. He makes the field knife with full flat grinds as well as saber grinds, so you can decide if you want to optimize for strength or slicing. And his knives come under your upper limit. Custom made knives are not out of the realm of possibility in your price range.
One other piece of advice for you to take or leave, is don't be afraid to buy a less expensive knife to dip your toes in these types of knives. When I was getting ready to pull the trigger on my first $200+ knife I bought a number of less expensive knives that had some of the features I was looking at to see if those features were actually what I wanted. For example you could pick up the Buck Selkirk, they can be found in the $50 range, and it looks like a solid knife, stainless, versatile sheath, and ferro rod. The steel is Buck's standard 420HC, and the knife is made in China, but it would allow you to try out a solid survival knife for less than you might pay otherwise. And who knows, it might be just the knife for you.