Okay, heres my 2 cents on the proposed mod.
The scuba tank has a higher oxygen content than normal atmostphere right?
Well if it does, than it will work along the same lines as NOS, which is used to increase the oxygen in the combustion cycle through the combustion of its cehmical compund. It then, theoretically, SHOULD add more hp, but i will guarantee that unless you set it up to inject in the RIGHT SPOT, your motor wont make full use of it, if at all. My car, for instance, has a mass airflow sensor (MAFS) that sends a signal to the ECU to increase/decrease the fuel mixture in conjunction with the O2 sensor. If i were to inject the oxygen sometime after the MAFS, the fuel mixture would then become too lean and the mixture burn too hot, which is bad for pistons/rings. Not all cars operate like this, some cars have a Manifold Absolute Pressure sensor (MAP sensor) that measures pressure/vacuum in the intake manifold and adjusts fuel accordingly. Of course, you wont be able to run more than (by my estimation) 4-5 pounds of pressure, depending on the CFM (cubic feet of oxygen per minute) youre going to be feeding the motor. You may be able to go to 10 pounds of pressure, but after the vacuum of the motor and the required airflow for the motor to function, you may only raise pressure by one or two pounds.
To put it simply: If done right, it will work fairly well... Provided you can fine tune it on a dyno.