In Barcelona, the dockyards are indeed at the foot of Las Ramblas. You can find them on any map of the city. However, the docks cover a pretty large area, so it can be a good 15 minute walk from the ship to the foot of that great promenade. It's also about a 10 minute cab ride from the old city.
In Venice, the ships dock near the Grand Canal, where of course you can pick up a water bus (vaproretto) to anywhere you want to go. Again, you can find the deepwater piers on any map of Venice.