On eBay there's an ad for a 500 gpd watermaker (US Watermakers) that
uses a brass General pump.

Question to co - most watermakers seem to use either bronze, s/s or
titanium pumps - yours is brass. what's the expected pump life in
tropical (85-90 deg) seawater.

Answer from co - We have watermakers with brass pumps that have lasted
20 years. The damage that occurs to the pump is primarily when the
pump sits in salt water. The system should be back flushed after each
use with fresh water thus eliminating this problem. Most of the units
we manufacture have stainless steel pumps.

What about using the brass pumps - what he says is true WRT corrosion
- typical operations would be 1 hr per day and back flushed with RO'd
water. And brass should be fine in distilled water.

But what about plain old wear from hours of operation. What about the
pickling chemical?