I had this happen to me with my OMC Cobra several years ago. I have
the 230 HP with 305 Chevy block.

I had started to pull the impeller to check the water pump when I
noticed the hose from the lower unit had slipped off the flange below
the water line. It was really difficult to get to with a screwdriver
to tighten the clamp, so I drilled a 1/2" hole in the housing to make it
more accessable. Worked fine and I've had no more problems. You have
to lay on your back and look up from the bottom to see if the hose has
slipped off. The problem you described is exactly what happened to me,
no water to the engine at all. Its an easy fix and I hope this solves
your problem.

Good luck, Norm