Stand on the Summer Street Bridge and look toward South Boston, and Fort Point Channel looks like a long rectangle, ending in a wall just past the Gillette plant.
In fact, the channel continues for a few more blocks, as a narrower drainage channel, ending at the Boston Water and Sewage Commission's Wet Weather Sewage Discharge Outfall Number 70.
You can stroll the Harbor Walk to what looks like the end of Fort Point Channel and admire the re-painted remains of a giant train bridge that crossed there before the Ted Williams Tunnel remade that area (and from which you can still watch the trains arriving at and departing from South Station):
To get to the very end of Fort Point Channel, though, you'll either have to continue into South Boston and come up the Kelly Bridge. or, if you want to see more of the underbelly of the Expressway, cross into Chinatown and take your first left past the Expressway, then follow the sidewalk that runs along the highway that will eventually take you past the old Herald plant and put you on Albany. Turn left on Traveler Street to cross under the Expressway, then right to follow the sidewalk along the channel to its end at the outfall.
If you take the Chinatown route, you'll get a better appreciation for the massive scale of the former Central Artery Tunnel project (look down and you'll still see some manhole covers embossed with that name) - and bonus views of what could serve very nicely as a battleground in some dystopian thriller:
But having reached the end of Fort Point Channel, look across the street and the bridge into South Boston, past the city maintenance garage, and imagine that entire area under water. Fort Point Channel was once just the entrance to South Bay, which extended south of what is now Andrew Square into Dorchester - and which once gave patients and workers at what is now Boston Medical Center waterfront views:
This map (from the BPL's Norman B. Leventhal Map Center) shows South Bay in 1890. The road crossing the bay at the bottom is today's Southampton Street (Swett Street back then); the next road up north across the bay is a bridge connecting W. 4 Street in South Boston with what is today E. Berkeley Street (Dover Street back then).