The boats at Boardwalk run between Epcot and DHS. The boat from Epcot stops at BW, the YC/BC, S/D and then down to DHS. When boating from DHS, the boat stops at S/D, YC/BC, BW, then lastly at Epcot. The boat has a dock right out on the boardwalk itself. The boats run up to an hour after DHS closes. But, there will be buses that will run from DHS several hours after it closes.
If going to DTD, from PO, I would opt for the boat. If heading to BW, I would take a cab if DHS is closed. I can't say how much it would be though...under $20 I would imagine. I would tend to use the cab for the ride back to PO from BW though if it's going to be late at night.
As far as heading to BW, looking for a night's entertainment....I love BW, I own DVC there. It is one of my favorite places in the world. But...unless you are planning on going to Jellyrolls, there really isn't all that much to do there. Yes, there is some family oriented entertainment on the boardwalk itself..jugglers, magicians, games. But nothing I would make a special trip to do...unless you are planning on spending some time at Jellyrolls. And like I said, if heading to Jellyrolls and leaving there late at night, there are buses that will get you back to your resort. But I'm just not sure what time those buses start running.
I would think that DTD would be much more interesting.