any good cleaner/degreaser will work,greased lightning,simple green,get some in there use some sort of brush and clean like a crazy man!!oh yeah,rinse/repeat,welcome aboard sir!
Is it grease or just mold, mildew, and grime? We used a gallon spary bottle and chlorine to clean up all the boats that came to our shop. Spary on the chlorine, let stand for 5 to 10 minutes and rinse. Repeat if necessary.
x 2 on the dawn,cheap,works,won't harm anything!
SB4 - Just a cautionary thought, but don't you run the risk of soaking the bottom of the fuel tank if you fill w/that much water?... I'm assuming the bow is only slighly elevated to drain normally.