For a lot of classes you would need to ask permission from the professor/lecturer, but I’m not sure how it works for individual lectures. I did audit a lot of classes but I was first a Harvard spouse and then working there myself, and you get an ID then, so it might be impossible if you dont have ID. I would start by looking at edx/harvardx online because a lot of really popular classes have an online variant for free there too. But that’s more common for, say, computer science than for deep sea biology. They also used to have in person lecture series for the general public, like Science and Cooking, where you just show up when the lectures are happening and if you’re early enough you get a seat, but I can’t find the overview of all the lectures like that online anymore. I guess you can find them at the events part of Harvard.edu. And you could always consider applying to Harvard Extension School!