Winter use of Mercruiser

[harringtondav]

I'm not sure, but I don't think the block stays completely full of water when not running and the heater element must stay submerged.

The block and head stay full, unless you have a raw water leak. And you'd know that. The exhaust manifold is also full, but wouldn't get direct benefit from the hot water in the engine. The exhaust manifold's water comes from the stern drive raw water pump. But if the block/head are warm, the manifold should share enough of the same heat. The raw water supply hose, and the power steering cooler - if equipped, could get a bit slushy. But that hose is down close to the 32 deg water.

If you insulate the engine box & lid, blankets - etc., you should be OK with a block heater. The only "marine legal" concern I see is having the electrical plug connection outside the engine compartment. Sparks and arching are the main concern.