While arguments can certainly be made that licensing might not have prevented this tragedy, and you can't legislate stupid, it is still a good idea and might prevent some future incidents.
I am always amazed at just how many people are running around out there that don't have the faintest idea about even the most basic rules of safety. Yet, if they have the cash, they are cruising. Some have been boating for many years and consider themselves "experienced".
How many times have you heard how "experienced" the skipper of that 34' Silverton was?
Licensing could certainly address that.
While it would be difficult to test the operation of all boats on the water, it isn't unreasonable to require that persons operating a boat over a certain size be required to demonstrate their skills before being allowed to run around out there.
Maybe they could issue a general license after written test for operators of boats under 21', and require a test on the water from 21 - 27, another from 28 - 35, etc..
Licensing fees could cover the cost of the tests.
A license could also be revoked for violations much the same way as a Driver License is, and operating with a revocation could be grounds for arrest.
It wouldn't solve all of the problems, but it might slow things down and send a wake up call to many who think that "the other guy" is the problem.
Unfortunatley there is no real way to measure the lives that are saved by enforcement of rules and good common sense.