A vessel was pressured into incorrectly loading slops into a brine tank. When cleaning the tanks and lines, there was insufficient water pressure. To check for contamination, fresh water was pumped from the brine tank through cargo lines connected by a fire hose. The hose end secured with rope came loose, spraying product overboard in violation of regulations. An investigation found inadequate planning, communication, and adherence to procedures by all parties involved. Changes were implemented to improve safety planning and empower crews to refuse unsafe work.