how difficult to change the raw water pump impeller.and does anyone have an exploded view?
thank you in advance

Looks about the same as any sea pump.

http://www.mercruiserparts.com/Show_...+Pump+Assembly

thanks Griff.
how often should you change the impeller?once a season or how many hours?

Every Other Season, WITHOUT fail.
Don't forget to drain the oil from between the fuel pump and water pump while you have it off. It will have Gas in the oil. Replace with fresh gear lube(outdrive oil).

The longest I have ever gotten is 2 seasons/130hrs. Usually I change it every season.

Have you used any of the supposedly "Run Dry" blue impellers? Any benefits over a mercruiser replacement?

I change mine every season. But, now I also change it after everytime I come back from the Mississippi river. There is so much sand and especially when you beach up for a couple nights. Twice now when I returned to the lake immedially following a river trip my impellor blew! One time, the whole casing cracked.

I don't have a sea strainer, though.

Never tried one. Probably would, if mine were not lasting as well as they do. I've heard they don't give quite as much water pressure.