Mostly just surface dirt, some fish guts, and general dirt build up. Usually just hit it with the power washer and purple power, but it didnt get it all this time.
I use Barkeeper's Friend liquid for everything from cleaning the stainless BBQ to polishing gelcoat...
That and a Magic Eraser will get most anything off--
And for baked-on fish guts and night crawlers and such, a little steam cleaner is your best friend...