I didn't come up with this. All credit goes to CenaxKikia on GameFAQS.
"ctf_2flood
Welcome back to 2fort, the most hated map ever. But oh no, something has happened. Theres been a flood! The water level is now risen above the bridge and just below the sniper deck. The entire sewer and first floor are submerged, and sniper battles are now USELESS against all the Michael Phelpses that are lurking in the briney deep!
Pyros, better stick to the respawn room, because its now up the swimmer spies and soldiers demos and medics to handle this dirty job! Guess what? Remember those grates that ventilate the intel room? No? Well I made those up, but never the less the water is SOMEHOW FLOODING THE INTEL AREA TOO! Its not just a race against the enemy forces, but oxygen too! And you know what, lets throw some trains in there.. one going up the spiral and... one that goes from sewer to sewer under the bridge."
swimming = slower than walking
sentries in intel and key choke points would be waaaaay too over powered