Since July isn't exactly whale season, I went into our cruise prepared to not see any whales. We booked a half day (2-3 hour) cruise in the afternoon. The reports from the week had showed good luck with a whale sighting more often than not.
We wound up spotting 3 humpback whales, one of which breached near the boat probably 40 times!
I recommend going up top because it is easier to see the whales on either side from there. You never know where it's going to show up, and you only have a few seconds to see it.
We also got into a pod of 200 dolphins, which was pretty spectacular in its own right.
The seas weren't too rough that day, but still there were half a dozen people who got seasick -- which made for a miserable experience. If you're at all prone to motion sickness, just take the Dramamine and spare yourself.