Like others have said, the west coast is rainy, not humid. In the Deep South, you have to worry about this stuff a lot more. I'm in California, which is much drier, but in either case I wouldn't worry too much as long as you can take an extra second to wipe your knife blade dry when you use it on anything wet.
I've had a lot of ZDP-189 in the past, and have been using K390 for about two months, which is a tool steel not unlike M4. Zero issues with corrosion either way. For what it's worth, ZDP-189 holds a "fine edge" for quite some time; I've heard M4 holds a "working edge" for quite some time. ZDP may take a bit longer to sharpen.
Between the two folders you've asked about, I'd be much more concerned with the style and weight of the knife. The Endura is about an oz lighter, and the handle may feel "cheaper" if you're not used to FRN handles. The GB is also about $50 more expensive than the Endura at Knife Center.
C90GRE Stretch 2 ZDP-189 British Racing Green
C28GRE2 Dragonfly 2 ZDP-189 British Racing Green
C12BK2W Matriarch 2 VG-10 Emerson Open
C81G2 Para Military 2 S30V
C81GPRGR2 Para Military 2 K390 Ranger Green
C223GPRGR Para 3 K390 Ranger Green
C90FPIV2 Stretch 2 Straight Spine, VG-10 Rit Dye'd Dark Apple Green
C223GP Para 3 S30V Green Canvas Micarta