For cleaning the exhaust stains off of the baot, I've found out that Zep Orange cleaner / degtreaser works well.

I bought a gallon at home depot for somewhere around the $9 range a while back.

I have some full strength in a squirt bottle for the exhaust, or any heavy stuff, such as grease, or oil. But I also have it diluted and use it to wash quite a bit of other stuff too. Can't remember if there are any precautions on the bottle (of what not to clean) though. I guess it would be comparable to simple green, but It seems to cut the heavy stuff a little more.

I've found it's a good all around cleaner that's relatively inexpensive.

Just thought I'd mention it..........................

I just found the BEST thing for soot:
Black streak remover. Spray it on good, rub it with your wash mitt and 99% is gone.

The ringo works great as a follow up. Afterwards, the transom will look like new.

I gotta tell ya, I've tried EVERYTHING trying to get rid of the soot. I just happed to srpay some black streak remover on the soot by accident. Didn't even rub it; I just sprayed it with a hose and noticed this big white spot!!!!!
Try some, it works wonders.

Hey stop following me around
Where did you find the black streak remover and is that really the name?

Yes that's really the name.
West Marine, BoatUS, or just about any other boat supply store.