Belt breaking is usually caused by misaligned pulleys or structurally weak
mountings that allow vibration. Both usually indicate a missing/bent/wrong type
bracket. If the pulleys have taken a bad set and worn side to side the belt can
be bottoming in the groove. That will cause them to wear out prematurely
because they slip.
First use a straight edge across the front edge to see if the pulleys look
right. Look for wear in the sides of the grooves. Then take a good listen at
idle speed with a new belt properly tensioned. Do you hear it popping and
scrubbing. Is there any strange resonance or vibration at various RPMs? Is it
running true, not wobbling ? I bet by then you will have seen the problem. The
toughest thing to find is a missing bracket since you may not be able to look
at one that is set up properly. Marine setups are sometimes cludged together
and they may have removed a bracket you would have in a car, to make room for
something else.