What would the advantage be over the monorail?More "water bus / taxi" services are coming in Tokyo Bay and related rivers. One of them is even going to Haneda Airport. The image below is a "new water bus station" opening this month. It's only 5 or 6 mins on foot from the nearest Yamanote Line station. Some new water bus lines will start next week and the trial of the new water bus station to Haneda airport will start July (next month).
A lot of related developments are coming.
This is the water bus route between the new water bus station and Haneda Airport.
A 5-6 minute walk from Hammamatsucho Station to get on a ferry as opposed to a couple of minutes walk to the monorail platform...
Although I’m sure the ferry experience would give you a different view of Tokyo. We spent a fair bit of our time in Hiroshima travelling to places by ferry and it was a lovely way to get around.