The Dash 8 300 & 400 have "ditching dams" at the window emergency exits. These deploy automatically...
Not true.
The 300 & 400 do have ditching dams, which are exactly what they sound like. They form a knee height dam at the foot of the door to hold off water in the event of a ditching.
The are NOT activated/deployed automatically.
The main passenger door ditching dam is like a baking tray mounted in a track, and in its stowed position sits in the ceiling above the door. To activate it, you slide it in its track until it hits the floor.
The forward right pax emergency exit consists of two parts, the upper part falls in your lap and the lower part hinges out from the cabin floor. To arm its 'ditching dam', there is a red handle to swing from one side to the other which locks the lower part of the door in place when the upper part is removed.