I'm replaying this old classic, one of my faves. I'm still early on in the game, in the village with all the houses. I've traveled underwater in the little submarine to the gallows and used the little lever to lower the manacles, then rode those up to the top, where the little prison cell is, with the man inside. I explored up there a bit, but now would like to get back to the submarine to explore the village some more, but can't seem to go back the way I came. How do I get back to the submarine?? I think there's still a schoolhouse I need to explore?

Check Graham's map here.
He has several in his walkthrough that are great and helpful.

There's a ladder going to the lower level to the left of the cave where the man is/was trapped, but I'm not sure if you'll be able to get to the sub again from there.