The last 2 boats I've owned I installed Prestone radiator flush Ts in the heater lines. Then to "winterize" I just had to remove the caps from the Ts & gravity would drain the heater core. Worked great for 7 years in a MC 205.
Then when I bought my Vride, my first fall project was to install those flush Ts in the heater lines. Seemed to go well. Till spring when my heater core popped anyway. I know I removed the caps & saw water drain. But when I got under the dash, pulled the subwoofer out, and looked at the heater core..... it was installed with the inlet/outlet lines on the top side of the box. Meaning that gravity would only drain the lines, and the core still had water in it. I replaced the core with a $29 Napa core, re-installed the core with the inlet/outlet lines on the bottom side, and have not had a problem in 2 yrs. We ride in the off season, when you need a heater. So removing the core was not going to work, but making it easy to drain worked out perfectly.