Solo Expediciones is the Punta Arenas boat operator, and the banner across the top of their site tells you the only thing that matters about timing: desde octubre navegamos a Isla Magdalena e Isla Marta. Outside the October-to-April window the colony is closed and nobody sails to it.
The half-day run to Magdalena and Marta is the standard trip — an hour ashore among seven thousand pairs of Magellanic penguins under George Slight's 1901 lighthouse, then a slow circle of Isla Marta, where the cormorants and the sea lions breed and where landing is forbidden. They also run the long ones: twelve hours down the Strait to Cabo Froward and the seven glaciers of Seno de Agostini, a full day of penguins and whales in the Francisco Coloane marine park, sea kayaking alongside humpbacks, and three-day trips on an exploration yacht.
Book through the website or on WhatsApp — the number behind the footer's "WhatsApp Link" is the same mobile they print as their cell line.