So this clearly wasn’t a mammal-watching trip…
We saw 5 species: Grey and Harbor seals at Ytri-Tunga point on Snaefellsnes (both species are always present).
Then at Husavik we took a whale-watching excursion with North Sailing. They have a 94%-96% success rate. Despite that statistic, on the afternoon tour we only saw 1 species: Harbor Porpoises (which at least I got a decent picture of). Then they gave us vouchers to go again for free the next morning. Only my sister and I went, and we got Humpback Whale, and my only lifer for the trip (I think): Minke Whale, which was too quick to photograph.
As Sabrina Bando so kindly advised me, white-beaked dolphins are best seen at the bay of Reykjavik (better seen from a boat, though we didn’t have time) than anywhere else around the island. Blue whales are almost guaranteed during July in Husavik while whale watching.
I also had my eye out for arctic fox which isn’t terribly uncommon around the island. Perhaps the best place for seeing them is in the West Fjords, specifically around the Arctic Fox center. Though a bit disappointed, I’m not too worried about it because I know I will see this species elsewhere. They can be seen sometimes in Snaefellsnes national park, Skaftafell National Park, or around Vik. Though Icelanders do not like them and will shoot them or call an exterminator if they see them anywhere because they sometimes attack sheep.
I think there is a tiny population of polar bears on one of the off-shore islands in the North, but you would clearly not choose Iceland as your polar-bear watching destination.
Otherwise, it’s an amazing country!