March is the most underrated month of the season. Whales are at their most curious and interactive after months of feeding. Leopard seals are hunting fledging chicks. The sun is setting again so the light turns golden, fresh snow returns, and the icebergs are as good as they get. Pricing runs roughly 20 to 40 percent below peak. If you can travel then, you get more wildlife behaviour for less money than at any other point in the year.
The objection people raise is that the penguins have gone. They haven't. There are fewer of them, and a first-time visitor wouldn't notice the difference.
The other assumption worth killing is that the ice is gone by March. Icebergs last for years, drifting and sculpting down, and the late season graveyards at Booth Island, Port Charcot and Pleneau are spectacular. A March trip is not a lesser trip.
One extra thing regulars mention: by March the crew have been together for months and the ship feels tighter knit. A few operators run into early April, where the weather gets more energetic but remains workable.