not sure on the 575 but the 525 also has a blue plug in the PS cooler

I would unhook both hoses at raw pump and shop vac them out. Should empty all coolers. Some don't have drains and should.

I've never seen that before???? Here is another pic I found. Derek explained where the plug is. It might be a Allen plug instead of brass hex head or petcock

I did a 575 yesterday. No plug on the PS cooler so I pulled the hose off the rear of it and it was holding a lot of water. If it is a factory unmolested merc 575, it will have the blue plugs in the bottom of the block