Water jacket, 115 Mercury

While hooked up to the muffs, I have water pouring out of the top cylinder at the spark plug.
I wish I could uplaod the video to show how bad it is, but unfortunately the site doesn't accept videos.
The still shots from the video will have to suffice.

My research indicates it's clearly the water jacket.
I've also read that the bolts seem to break off easily when trying to remove them except when an impact wrench is used.
My questions:

What is the best way to remove them without breaking?
Can I remove them without lifting the powerhead? The bottom 3 especially.
Is there any alternative to replacing the whole water jacket seal?

Electric impact wrench from a rental place.

You also may want to apply some penetrating oil to the head bolts & let it soak overnight. You can make your own with automatic trans fluid & acetone 50/50. That is as good as the stuff they sell in a spray can.

Got the water jacket off without breaking any bolts.
What is the best way to clean it?
Should I worry about the appearance of the top 2 cylinder heads compared to the bottom 2?

If you want to cut the scum & lime deposits, a cheap toilet cleaner like 'the works' will do that. Just don't get it on you (esp eyes) or anything you don't want stained.

The leak was a failed water jacket gasket.
I'm in the process of cleaning the cover and the frame around cylinders.
The gasket and sealer are really stuck on in a few places.
I have the new gasket and will replace once it's all clean.
What is the recommended sealer to use with the new gasket?

Unless you need something to hold it in place, I find that just having clean surfaces is enough to let the gasket do its job.

Sometimes I have used cut off screws as studs to align a few holes & hold gasket in place while assembling. This is esp. helpful if it is floppy.

How clean does it need to be?
I've done the best I can with a putty knife for the thick stuff and a razor blade for the rest.
It's smooth to the touch but definitely doesn't look like it.

You may want to go over it with some very fine grit sandpaper (like 800 grit) on a flat block and see if that polishes it up any without removing any excess material. Make sure and keep it flat and even, do not round the sealing surface.