Spark plugs corrosion

[cmb1998]

I have a 175 Sportjet that I rebuild last spring. It's got about 50 hours on it. I put new plugs in it at ~15 hrs and then cleaned them up with fine sand paper at around 30 hrs. I pulled them today and they have quite a bit of corrosion on them. What causes this? The sportjet doesn't have thermostats, and always runs cool. my temp gage never moves.

[cmb1998]

It doesn't feel like carbon as when I sand these I can see the metal is pitted.

[FMP]

What make and # plug. COuld be a bad batch ? Do you run in salt water? Buy a fresh set and hope for the best.

[TEXAS20225]

looks like water in the cylinders