That's the problem with "marine grade" these days... it just means it costs three times as much. Sounds like it couldn't handle a puddle in a dirt parking lot.
My old boat had the same issue with silt. I ended up using a basic Rule 2000GPH pump with a big external strainer. That combo ran for two seasons in muddy river water without a single clog. The key was keeping the silt from getting to the impeller in the first place.
Man, I had a buddy who tried one of those cheap "marine" pumps and it died on him first trip out. He ended up borrowing my old Rule pump (the one that's been sitting in my garage for years) and it ran like a champ through all that muck.