The water feed line in the drive is not a sealed path. Pressure from a garden hose will force enough up to get to the hose. When the boat is in the water the water pickup hose is submerged in the water, that's why it works when it in water.

I went through this the first year I bought my boat. Bought one of those winterizing kits that came with a 5 gallon container. I placed it on the swim platform (higher than engine) and proceeded to over heat the bearing in sea pump waiting for the antifreeze to get sucked in. It didn't work.

You need to pressurize the antifreeze to force into the pump.

Ill try pressurizing with a Water Puppy pump and see what happens.

It will also help if you tape over the water pick up holes on the drive