That seems to assume that many people would hijack plane if it were easy, so raising the bar decreases the incidence of hijacking. That assumption is not very plausible, as evidence by the fact that it is very easy to hijack a bus, yet bus hijackings are as rare as plane hijackings.
However, if you make a different assumption, namely that it is only a very small but determined group of people who want to get into the hijacking business - security measures as we know them are unlikely to reduce the risk of hijacking. Their only effect is to increase the cost of the hijacking operation. Sort of like going through the Bay Bridge - raising the toll does not prevent people from going through (it's called high price elasticity).
I would argue that the best way to reduce the likelihood of hijacking is to reduce the number of people who would be determined enough to effectively carry it through rather than through raising the entry barriers to this occupation. However, either proposition is difficult to test empirically.
As to the seat belts - I wear them almost all the time, but this is mainly due to the possibility of turbulence. If the plane suddenly looses altitude (a rare event, to be sure) everyone inside becomes a flying object, unless attached to his/her seat by the belt. However, there is a very little doubt in my mind that the "in the unlikely even of landing on water" part of the aircraft safety message is mainly to make the passengers feel good. Just think what would happen to your internal organs (brain, heart, liver, etc.) traveling at 200 mph when you body attached to seat by the belt comes to a sudden stop when the plane hits water.
wojtek