By foot, shuttle, or gondola.
There are many enjoyable ways to get to The Loeb Boathouse. Simply walking from 72nd Street and Fifth Avenue is always an option.
If your attire and your inclination are not fit for a stroll—the Boathouse Shuttle makes stops along upper Fifth Avenue. The shuttle* can also stop at 80th Street and Fifth Avenue to meet or drop off guests who have parked at the 24-hour Metropolitan Museum parking garage.
Of course, you may prefer to arrive in classic New York style by horse and buggy. And there is an actual Venetian gondola on the premises—not really a means of transportation, but a fun way to explore the lake once you have arrived.
*Both shuttle and restaurant are handicap accessible.