what is the best way to remove water lines and green scum on the bottom of a boat?

Buy a 2 gallon garden sprayer and pour in 2 bottles of toilet bowl cleaner( i use Sno-Bol). Top off with water, pump up and spray all areas liberally and watch the green change to white.

Be careful not to get in eyes or on skin as it is acid.

It also removes wax, so keep that in mind.

Let me add to buy the cheapest toilet bowl cleaner you can find. I tried Tidybowl and it didn't hardly work at all. Got the no name stuff and it worked great. I think the cheaper stuff has more acid. Tidybowl may work good on toilets, but doesn't cut it for boats:drink
Eddie:

with any acid based cleaner ,you should wax the area w/a good marine grade wax.if you do not,the stuff will come back and be even harder to remove the next time.

Muratic acid. Its about $5/gallon. Use the same method in the pump garden sprayer. Rinse very well and make sure you are up wind.

Griff, I assume you clean that boat each time you take it out of the water? Acid & wax?

On & Off work wonders. Just brush it on and rinse off. Just make sure you use gloves and eye protection. Strong stuff. I hear they also have a gel which is supposed to be more bearable.

I maybe acid it to get the river stain off once a month or so. Most of the time I just wipe it down when we pull it out. After a long day on the river it gets a little yellow/brown, but I figure its going back in the river the next weekend anyway.