Photographing open-ocean cetaceans can involve a lot of swimming.
For instance, to be accepted into this group of sperm whales required what seemed like hours of swimming as hard and fast as I could. In reality, it was probably only 10 minutes, give or take, but swimming for 10 minutes in ocean swell with the objective of catching up to and overtaking a group of whales is exhausting...especially if you keep doing it over and over again.
But on those occasions when whales slow down just enough to let me swim side-by-side with them for a while...it's well worth the effort.
Note: Photo taken under permit.