I can only locate three blue plugs to empty the water out! Last year when I winterized it, I could have swore there were at least four. I found two on the passenger side and only one on the driver's side.
If its mercruiser ski... Mine has 6. Starting from the bottom- 1 for fuel pump cooler, 1 on each side of block, 1 on each manifold, 1 just past the recirculating water pump/thermostat. Get a flashlight and stick your head down in there