When they first arrive in the spring you can see their vertebrae as they arch their back for a dive. After eating all summer, you can see how much they've filled out by the time they pass through on their way south in the fall. When they arch for a dive they've fattened up enough that you can't see individual vertebrae anymore.
It's an amazing experience to paddle a kayak out into a calm bay and sit for an hour surrounded by a dozen humpbacks, or to sit in a calm stretch of ocean with whales surfacing as far as you can see in every direction. They'll come within ten feet of a kayak, and you know they'll never bother you. They just seem curious about what's out in the water with them.
They engage in bubble-net feeding. A group of humpbacks circle underwater beneath a school of herring. They let out bubbles and build a wall of bubbles around the herring. The whales then swim up inside the bubbles and break the surface with their mouths open, feasting on the trapped herring. It's wild to see a ring of bubbles appear near you on the surface, and then see half a dozen whales rise up out of the water with their mouths open.
My favorite part of being close to whales is listening to them breathe. You can hear the size of their lungs from the sound of their breaths.
If you've never been close to whales, try to get on a whale watching tour. They're just incredible to see in person.