Right, just because it's the government and you don't know what the vessel's up to, it must be something stupid. The coast guard's business is icebreaking, northern resupply, navigational aids, search and rescue, ship inspection and standards, channel markers, mapping, lighthouses, navigational hazards, and weather advice, and for those purposes operates 33 coastal radio stations, 15 maritime traffic centres, 55 search and rescue stations, 56 major vessels like the one you saw, 35 helicopters, 74 rescue vessels, and 4 hovercraft. Ya coulda looked it up, it's not secret. Inching back and forth along the coast suggests some sort of mapping or charting activity. The coast guard maintains some 13,000 channel markers, so obviously it has to know where the channels are, and they've been known to shift. It could also be something as simple as training new crew in navigation and ship handling, you'll see naval vessels behaving the same way when that's what they're doing, I've seen them in the Saanich Inlet. But really, you probably could call the nearest coast guard station and ask if you really want to know, instead of leaping to the conclusion that they're up to something whacky.