2007 Marlin 300 >>>> I drain the water tank and the hot water heater .... In my years of servicing hot water tanks / heaters., I don't open the relief valve if I don't have to. The odds are it is not going to reseal when you close it .. I installed a shut off valve and a tee with a drain on the Water tank. . " TANK >> SHUT OFF >> TEE/ DRAIN>>> PUMP AND HOSE FEEDING THE SYSTEM.... After draining the tank, I shut the valve which will isolate the tank from the system .. Then I connect a Compressor to the Tee/Drain and blow out the heater , pump, faucets ( all of them), Everything in the fresh water system distal of the tank. As I go, I open and close faucets and the Hot water tank drain valve, alternating between each... I go back and forth till i am satisfied that all the moisture is blown out from the fresh water system . I have a similar set up for the wash down, live well, A/C unit and Generator. I Flush the A/C and generator with fresh water and then with Anti Freeze. I Drain the shower sump, and add some antifreeze, and throw a couple gallons of antifreeze into the head and flush. I throw some antifreeze into the hull just as a precaution for any water that may be still not drained.
I made up Brass Tee's That are installed between the thru hull shut offs and the corresponding systems. The same sequence as described above. The Bull side of the Tee has a brass Male pipe thread to male garden hose thread fitting. I normally seal this off with a brass hose thread cap and hose washer.. . I have an adapter for my compressor that ends in a female garden hose connector . I remove the cap from the Tee , Shut off the thru hull,, connect the compressor, and blow out what ever system I an winterizing . All I can say is it works well ... For the AC and Genny , I do the same but flush with a garden hose , rather than compressed air. Then I have a container filled with antifreeze, with a female garden hose outlet and connect that to the fittings on the AC and Generator lines . I run both till "pink" comes out . No pumps are needed. the antifreeze pumps thru the strainers and pumps of these systems. After I am done I open all the thru hull shut offs.