RE: Raw Water Pump Impeller
WE usually install a Y strainer if the sea water pump feed is 1 inch or under.
A failed impeller is no big deal, one that fills and plugs passages in the heat exchanger and exhaust is a bigger deal.
The Y strainer is AFTER the impeller pump , and when the pump fails it will catch all the bits and pieces that usually shed off .
This is easier to monitor than the impeller it self , and will show the first bits before the entire impeller fails.
In shallow water its cheaper/easier to monitor than simply changing out on run time.