I would first try to drain using the clear drain tube into a bucket. If I wanted to flush the system I would detach the hose going the hot water heater at the heat exchanger, attach a hose from the heat exchanger to a bucket, then attach a fresh water hose to the water heater hose, then have fresh water push the system fluid out to the bucket. After clear discharge is achieved, detach a hose at the water heater and allow to drain to bilge.