An extreme loss of braking friction due to heavy rain and the water depth on the ungrooved runway, which resulted in viscous hydroplaning. Contributing to the accident was the operator’s inadequate guidance for evaluating runway braking conditions and conducting en route landing distance assessments. Contributing to the continuation of an unstabilized approach were 1) the captain’s plan continuation bias and increased workload due to the weather and performing check airman duties and 2) the first officer’s lack of experience.