We went out with San Diego Whale Watch during week of May 1st. We knew that it was inbetween seasons for whales, and we probably wouldn't see anything, but we did! We saw 2 Humpback Whales, and about 500 dolphins. It was truly great. We weren't expecting to see whales just due to the season, and then saw two! The Captain and biologist on board did a great job of searching them out. The dolphins were so cool. There were about 50 of them bow riding along with the boat, jumping out of the water and diving back in. So great.
I recommend bringing a sweatshirt & windbreaker. If it's a cooler day on land, it's even cooler out on the water, and you also have a strong wind as the boat is moving along. There were people on our boat in long flowy skirts, and they were suffering and cold. There is a downstairs area that is somewhat indoors where you can get out of the wind, but jeans, sneakers, are going to be best. Also bring a sweatshirt and windbreaker.