After much consideration I have decided on a knife. Thank you to all who contributed advice. I had no idea there were so many good knifes out there for the purpose!
Honorable mention goes to the Boyd knife. #1 with Practial sailor, made of a metal that will never rust and looks to be a life-long tool.
Honorable mention also goes to the Wichard entries, both the model posted by Jackdale (Post#2) and the "one hand" model which does look easy to open with one hand.
In the end I decided that the REAL need/intended use for the knife would be to cut something free in an emergency. To that end I decided that even though there are some great folders out there that I would go with a fixed blade knife. If I'm trapped by my tether hanging over the boat side while wearing my bulky helmsman gloves, or if my foot is in a loop of anchor line going overboard I can't be sure I'll correctly fumble the folder out. The rest of the features, like marlinspike and shackle key, I'll have to live without on this tool in order to guarantee it'll work when needed..
In fact, I never even knew what a marlin spike was before this thread, so I think I can continue to get by without one at hand. I may yet get a Myerchin P300 which has the marlinspike and pliers on it as a tool, but not for emergency use.
Since I was after a fixed knife that I could be sure I'd get to, and would work in an emergency, I found that kayaking knifes and scuba knives offered the best selection. There are many good options that are meant for easy access, even with gloves on (especially scuba knives), and that don't rust.
This is the knife I chose. Wenoka "Squeeze" Lock Titanium Knife, Tanto Tip
It's got a positive locking sheath with quick access (even with gloves). 4.7 out of 5 from 54 reviews. It's blunt tipped with a sharp regular side, a serrated side, and a line-cutter notch. It's 100% titanium (not just the handle) so it's light, will hold an edge and will never rust. $53 shipped. No shackle key or malinspike, but looks like a trusty tool that'll be accessable and will work if you NEED
it. I'll review it here when I get it and after I've used it a bit.
PS In close second was the NRS pilot or copilot knife. They had mixed reviews though on their retention system. Some said it had metal springs that rusted and others lost their knives. Also their titanium version is $120. I bet that the oxygen wrench on the NRS knife would make a decent shackle key though...
PPS Here is another titanium knife like the one I chose. Cheaper, smaller, and still highly rated. Less than $40 shipped and it's 100% titanium!http://www.leisurepro.com/Prod/BLRKM...-header-BLRKMT