Hi Sally,
Sorry for the delayed response, I was getting all the nitty-gritty details..
So, during the winter months the only afternoon whale-watching tour available is at 13:00.
Now the city tour is about 3 hours long from 09:00-12:00 so you would be able to do the whale-watching in the afternoon.
The Blue Lagoon excursion starts at about 10:00 and you won't return to the city until about 14:00 so you'd miss out on the whale watching on this day.
Kindest regards,
Julia