I guess the boat will weight something around 7kg.
If you want it at around 20 km/h you will need around 250-300W outgoing power of the motors so ~ 25A without taking the motor efficiency into account.
With these motors (check the efficiency of the 850 slightly above 50%
) you will probably have 35-45 amps getting out of the batteries, so max. 15 min flat out run time.
Getting better motors will maybe enlarge this period to 20 min.
Realistic (in my experience) you dont run the boat full speed at all time. So you can calculate with around double run-time.