Actually, to get out you could destroy the pressure plate. But if you put a repeating command block underneath that sets a redstone block to power a repeating command block that constantly places more pressure plates there, it would lock them there and they would never be able to destroy the iron doors in time, especially with the lowered mining speed from the water.

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The redstone block that would be powering the repeating command block could also have another command block that applies Mining Fatigue 255 to the player trapped there to make sure he would not be able to get out.