This is the fancy implementation.

• 2 Doors, any variety that doesn't leak
  
• 1 Liquid Sensor
  
• 1 Not Switch
  
• 1 Drain (more recommended for high use areas)
  
• 2 Small Floor Buttons or Proximity Sensors

Place your Buttons right next to the door, one on either side. Wire them to the top node of the door.

Place your Drain and Liquid Sensor on the floor next to the outside door. Place your Not Switch anywhere you like. Wire the Liquid Sensor to the Drain and the Not Switch. Finally, wire the Not Switch to the top node of the inside door.

This is the simple implementation. Those with Wiring Mode don't need to use chat commands.

Press Enter and then type:
/admin
/spawnitem drain X
/admin

X is the number of drains you want to use.

Just remember to close the doors behind you.
You have two choices.

1. Make a depression between two doors and manually pump it dry before opening the second door.
   
2. Make a hole in the floor. It should extend to the bottom of the planet.

• Cake for Žižek, for suggesting I make this guide.
  
• Apex-prey, for suggesting the second alternative to /admin.