I put a swimming pool (water depth 4) right smack in the entrance. A small warning to anyone who decides to do something like this, make sure the dwarves have another, longer "path" they can use instead of going through the swimming pool when they need to haul things/build things, since swimming apparently forces them to drop anything they're carrying and to cancel whatever they were doing.
My main plan is to block off the fort's Outer entrances with water, so anything that tries to attack my fort will need to be able to swim to get to it. In case they CAN swim, I have a few drawbridges that will block all access to the fort. My idea is to train my militants to become good swimmers, so they'll be able to bypass the water entrances and attack from the water. No idea if dwarves can cross through water after they're trained up in swimming.
