I have no experience of your engine
, but in general the most common reson for an impeller failing early is running dry.
Other common causes are an incorrect impeller, and scored or damaged face plate or pump body.
Make sure you find the broken bit of the impeller, to ensure its not clogging up the cooling
I would change the impeller more frequently. This should be done every 12 months or 100 hours of running, but impellers are cheap
compared to rebuilding engines, so more often would be sensible till you find the cause of the problem.
Check there is adequate water
from the exhaust
at all angles of heel. The water pump should feel cool.