If you were going to use a scuba tank I'd suggest going into a sewer or underground telephone compartment. That way you wouldn't have to deal with the debree. I'd prefer the start of a sewer run VS half way. It'd flood much slower. After the initial blast passed over and the sewer had totally filled with water I'd suspect you'd have to rocket to the surface. You may be under a critical amount of water. Sounds like a James Bond stunt anways....I'd prefer taking off in a powered parachute (ceiling of 5000 ft) plus the incoming wave would likely cause the incoming air to send you higher.
-NIM