We just got back from our cruise we took last week. Our whale watching excursion started at 8:00 AM. We saw 12 whales!! The guide said we lucked out because it had been overcast just before then, and cleared up by the time we got there. We were told they tend to be seen more when it has been overcast.
We were on the Radiance of the Seas from August 6-13. On our first morning out, a cruising day, my husband got up early (6 am) and saw many humpback whales (he guesses about 15 or so) at a distance. This happened over the course of an hour or so, and by the time I got up and out, around 7:30, I saw several myself. When we stopped in Juneau the next day, we took a whale-watching tour that left at 1:30 (after taking a shuttle to some bay, but I don't recall the name exactly). We saw 4-5 whales at that time (including a mom & baby); must've been around 3 p.m. This tour promised a sighting or $100 back. All our days were sunny, by the way.