Just like at your airline, the FAA certifies EVAC performance for each airframe type based on a regulatory criteria. FAA POI's work with each airline to develop training standards that meet regulatory guidelines. Some procedures may differ from airline to airline as long as they meet regulatory requirements.

That said, every flight attendent I've ever talked to is trained to determine the usability of an emergency exit before opening it or sending a passenger through it. Said a different way - they have to use judgement.

Not opening doors below the waterline when you have at least 6 usable exits above the water is an example of good judgment in my book