Marine motors usually make torque and HP at lower RPM since there is no transmission(most of the time!) to help it get up to speed. Also, boats have more of a constant load requiring different HP and torque curves. Running at a high RPM for long periods of time has the same affect in both applications - drastically shorter engine life!
Many people use car/truck motors and seem happy. But there is always a performance upgrade to be had with a marine motor - like stronger out-of-hole. The biggest difference is the cam specs.