This website might help you for getting to St John:
There is a section near the bottom titled "Ferry Schedules/How to get to St. John from Ship " and it is very detailed, including ferry schedules and prices.
I don't believe you can go from ship to beach on St John in one hour. Assume about 20+ minutes for each mode of transportation (taxi, ferry, taxi) plus wait-times, ferry boarding time, etc. I would say slightly over an hour assuming you walk off the ship at the right time.
Also, it is called "St John" (singular) as "St Johns" (plural) is a city on the island Antigua.