So far, when I have done raw water impellers on my big blocks (mainly saltwater use BUT saltaway every time)...I also find the composite housing is etched and I replace that at the same time. I already have the Hardin SS sea pump (front).
The composite impeller housing/body costs about $40 and the Hardin Stainless Steel housing/body is about $160 so after doing this 4 times (on each of my twin engines) the SS would pay for itself...if they are impervious/resistant to etching/damage/leaking/etc...

Are the SS that good?
Other than periodic maintenance/replacement I have not had any leaks/failures from the plastic/composite impeller body.

I've been using my original Merc plastic like body since 1995. Having said that it does share duties with a brass Jabsco unit that I swap back and forth come rebuild time.

I had to completely rebuild my Hardin gen 4 SS this year after having it a little over 10yrs (original cost was $369 in Feb 2012). This spring, one of the bearing races split completely around the circumference, caused both seals to wear out and grooved the shaft; leaked like a sieve. Replaced all the bearings, seals as well as the shaft, impeller, wear plate. Cost me about $150 for the master rebuilt kit plus another $150 for the shaft + another $30 for shipping. Had I caught it sooner, it may not have worn out the shaft. The housing was fine, but I'm not sure I would consider this a lifetime pump. Up until this year I just replaced the impeller every other year. I think current cost is ~$400. Mine has only seen fresh water.....

How bad is it etched? The Merc housing should be fine for a while unless it runs dry or the housing breaks.

The Jabsco brass has been awesome for me. Very reliable and easily rebuildable. Very little wear .
Got one left for sale too .

All of them get some etching. Unless there is water flow problem or lack of water pressure, then its really not an issue.

I'm fresh water and back to using the composite. Complete new housing $100 every other year. Used the Hardin for 2 seasons and was looking at replacing the plates. I also have to hang upside down over the motor just to replace. Its heavy and just not worth the time and pain in the rear, so the stainless is sitting on the shelf.

Etched slightly and I do keep the least etched old one as a backup...but for $40 (cost of body alone) I figure why risk it.

For these reasons I will probably just use composite.

I have used the Merc plastic, the Jabsco brass and the Hardin SS. I have had less maintenance with the Hardin SS pumps than the others. I personally think they are worth the time and cost.