What master sergeant said about the weight definitely has merit. Approximately 70-80 pounds is worth .10 seconds in the 1/4 mile.
Regarding the pump gas, that depends on a lot of factors. Ultimately it comes down to (with any engine): when air is compressed, temperature increases. All mechanical superchargers heat the air. The efficiency of the supercharger is the ratio of how much heat is added to the air during the process. This is Intake Air Temperature (IAT). IAT is then added to the ambient air temperature and to the heat generated during the compression cycle. Then there is the heat that is transferred to the intake charge from the internal surfaces of engine. If this temperature is greater than the flash point of the fuel, it preignites.
If you do go the supercharger route, a centrifugal is better suited to jet boats. A Positive Displacement (PD) blower makes so much torque down low in the RPM range, it can overwhelm the jet pump impeller on the hit. The centrifugal generates boost where it can be best utilized
Regarding, Do you "add a blower to the current 454 motor thats in the boat and call it a day?" That really depends on what is in the 454. At the very least, you need to double-keyway the crankshaft and add a billet hub or damper.