Since this thread may serve as a resource for people searching on the forum for info. on Spyderco H1-steel knives, where the H1 steel is allegedly rust-proof, I'll pass along what I've been told and a small bit of personal experience.
1.) I'm told they are very rust proof (but note: not all Spyderco knives use H1 steel), but you may on occasion find a bit of rust on the blade. Someone claimed it might be due to residue from the (non-H1) bit used to engrave things into the blade, and I think I can see some evidence on mine. I'll post photos taken today with my iPhone 7Plus and resized to 1024x768 (or similar) to show you what mine looks like.
Speaking of which, mine has been on 4 live-aboard trips IIRC, and more diving besides that. It's an old Spyderco H1-steel Salt 1. I don't rinse if off between dives, it's never been oiled or sharpened, I seldom use it (though it seems sharp), and basically it gets rinsed at the end of a live-aboard trip like the other gear.
2.) If you're willing to blow money on a Chemistry experiment, one person found a way to corrode the blade. You know those big chlorine tablets sold in plastic buckets for chlorinating swimming pools? Well, somebody said prolonged direct exposure to those can corrode an H1 steel blade. My knife cost around $60 (IIRC) years ago, and I'm not curious enough to try it.
3.) My old Sherwood Avid BCD right pocket had a small outer pocket that fit it nicely; for my newer BP/W setup I bought a Dive Rite thigh pocket, and the top has a zippered pocket that fits it okay. I have a Trilobyte on my chest webbing.
I've had a couple of those ~$40 'stainless steel' dive knives, and they rusted badly with much less salt water exposure. The big question is, since cost is similar and rust-proofness a pro. for both,
how does an H1 steel knife compare to a titanium knife, all other things being equal? I've never tracked down a definitive, convincing account answer
that.