This movie demonstrates the latest version of our swimming training room being activated. The marksdwarves in here were stationed here, and then the door was locked.
The logic gates over to the right, which you can see operating during the movie, are four AND gates, one for each pressure plate. Each one has a pressure plate as one input, and the yellow lever as the other input. Their output is each connected to one hatch in the ceiling (there's a water tank above which is kept filled by a couple pumps). (Each pressure plate is controlling, through the AND gate, a hatch directly above it)
The pressure plates are set to 0-2 water (0-3 would get us up to 4-5 water if we weren't using AND gates, but we need it to shut off quicker due to the delay in the AND gates).
These particular AND gates are based on the ones I posted on the DF wiki, but with some modifications and optimizations to make them respond to a shut-off signal from the pressure plates as fast as possible. They're based on the 'faster' variant, first, and the pressure plates in the gates are set to water 7-7 for faster shutoff, and the pressure plates are linked to the doors rather than the hatches (helps it respond faster by eliminating the 7/7 water in the door tile, so the pressure plate isn't refilled by it).
The chief limiting factor is the delay inherent in pressure plate transmission of signal to hatches and doors (they have the same reaction time).
The main reason we built AND gates into this system, though, isn't seen in this movie - Before we let the dwarves out of the room, we flip the yellow lever back to OFF, which locks the ceiling hatches so that the pressure plates cannot open them while dwarves are leaving (Without the AND gates, one or more of the pressure plates tended to activate if enough water was lost out the door, resulting in much of the room being filled to dangerous 7/7 levels).
Note: 6/7 water is perfectly safe, but anyone in it or in 5/7 water will be swimming. You only start drowning if you're in 7/7 water.
( The logic gates are at http://www.dwarffortresswiki.net/index.php/User:SL/Logic_Gates )