-- Last Updated: Feb-15-13 9:28 PM EST --
... was set forth in the last sentence of the OP:
"I was wondering if . . . spare paddles change with the type of water, or if there is any logic to the selection of a spare?"
My answer is, yes, there is always a logic to the type of paddles I bring on a trip, whether a day trip or an overnight trip. None of my paddles is ever a "spare" in the sense of being a cheap piece of junk I would only use as a last resort if I lose or break my main paddle. All my paddles are quality paddles that I like to use in different circumstances, almost always during the course of the same trip.
Therefore, I pick paddles for my trips depending on the type of water (depth, current, flat, white, rocky, beaver dammy), the pace of paddling (gunkholing, cruising, exercising, racing), and my position in the canoe (kneeling, sitting, standing). If I'm only paddling FW, I'll take a straight and a bent paddle. If rapids are part of the trip, I may take a WW paddle. If I want sit 'n switch speed, I'll take a smaller blade. Cruising, I'll take a larger blade.
When kayaking, I apply the same logic, though my number of paddles and shapes is much less than those of my canoe paddles. I'll typically take a soft-bite blade for easy cruising and a big-bite blade for speed and acceleration. If I'm paddling lakes connected by bony streams, I may take my beater ABS blade to handle the rocks. If I had a wing paddle, which I don't, I'd put that somewhere in the paddle rotation depending on the type and purpose of the trip.
I can also see a completely different logic of always staying with the same shape blade. In that case, I might take two similarly shaped blades made out of different materials, or one being a one-piece and the other being a two-piece. But that's never been my logic.
I didn't think storage position was part of the topic, but I generally don't like anything on top of my front deck in a decked boat.