I put my boat in the water for the first time this season yesterday. This year was the first year that I winterized just by the book (pull the drain plugs, open disconnect the crossover hose, and take the cover off of the impeller). I usually flush with antifreeze THEN do the book's procedure - overkill. I was terrified of a scene like yours... Everything was OK for me though.
2018 Moomba Craz | Autowake 2.0, Zero-Off, G6 Pumps | Captain Blue/Dark Graphite/Silver Flake | Enzos + Lead