While I don't have fishing experience with twin Honda 90's on a 25CD, I do have plenty of experience with twin Honda 40's on a CD 22 and a tiny bit of experience with twin Honda 135's on a TomCat. Bottom line, with one engine at idle in calm winds and no current, you'll probably troll at around 2.3kts maybe a tad less. I usually wind up trolling around 3-4kts (especially for coho) so trolling on one engine is no problem at all. In the rare cases where one may want to troll at a slower speed (say for sockeye in a lake), a drift sock, sea anchor or even a couple of 5 gal. buckets tied off to the midships cleats will slow you down enough.
I've spent a lot of time trolling W of Neah Bay (from Cape Alava to the S to Swiftsure to the N and as far west as Blue Dot). The condition there should be similar to what you'll see west off of Vancouver Island. In my 22, I'd be able to go out at 14-22kts if the wind chop was less than say a foot or so. Once the chop was up to about 2 ft, I'd be around 8-12kts and above that, maybe as low as 6-8kts. Keeping the bow low does help one plow through waves but if you're going fast enough to get any air under the hull when going over a wave, you'll really notice how hard a nearly flat bottom boat hits the water on it's way down. A deep V is for sure nicer in that case but that case can be avoided by slowing down.
As others have pointed out, the upside of the nearly flat hull is great fuel mileage - about 2x what you'll get in a comparable size/weight deep V. Also, the C-Dory will get up on plane at much lower speeds - 12-14kts depending on whether you have perma trims or trim tabs. This gives you a much broader choice of operating speeds relative to a deep V (which often can go either hull speed or fairly high planing speed but which don't operate well in the gap). Also as Dr. Bob points out, the C-Dory hull is particularly nice in following seas as large swells/waves simply roll under the hull as oppose to creating a broaching problem.
In my experience there are really no conditions in which a C-Dory can't handle the water as well or better than a comparably sized deep V. You may not get there as fast as some deep V's but you'll be as safe as any other boat and you'll burn less fuel than most. In some (rarer) conditions, you may have an advantage in a C-Dory due to the relatively low planing speed. I've been as far as 30 miles off shore in my CD 22 and regularly took it 8-15 miles off shore in the summer time. The CD will serve you well for salmon fishing. If you have any specific questions feel free to send me a PM. If you want to talk over the phone, PM your phone number.
