Is the Kanto area rail pass the best for us

Just wondering if the 3 days is 24 hr periods or calendar days. We arrive in Tokyo on Friday night and then head to Yokohama to board our cruise at lunch time Monday. Is this considered 3 or 4 days?

The Kanto Area Pass is valid for 3 calendar days, so it won't cover your entire stay. On one Tokyo-Yokohama leg you'll just have to buy regular tickets which are not expensive if going by regular train.

Btw, the Kanto Area Pass may not pay off for you unless you'll be doing some sort of sidetrip on that weekend (Nikko, Kamakura, Mount Fuji).

The pass runs over calendar days; so if you were to activate it late one day, that would still count as one day. So your itinerary contains four days.

How do you arrive in Tokyo?
Where in Tokyo is your hotel? Is it near a JR station?

Is the Kanto area rail pass the best for us?

That depends on your itinerary.
Usually a JR Kanto Area Pass does not easily pay off in trips inside Tokyo + one trip to Yokohama.
This pass is not valid for your non-JR train rides to or inside Yokohama.
At least three companies are operating lines which connect Tokyo and Yokohama: JR-East, Keikyu and Tokyu.

If the cruise ship departs from Osanbashi terminal, you can either take a bus from Sakuragicho Station bus terminal or have a Minatomirai Railway ride to Nihon-odori Station and walk from Exit 3.

Thanks for the advice. We arrive Friday night around 7.30. We are staying at a hotel in a Ginza quite near Tokyo station. We are booked on a cruise leaving Yokohama on Monday evening although we can board on Sunday and spend Monday either in Yokohama or in Tokyo.
Any advice on activities in Yokohama would also be appreciated.

Only Yokohama? Then do not buy the pass.