I have volunteered at and helped coordinate one waterstop at the country music marathon. I'd alot of work, and planning can be stressful. Do you have to decide how many waterstops, where, etc? Are you coordinating a sports drink and water?
The main gist is you need lots and lots and lots of volunteers. At CMM, groups volunteer for each individual waterstop, and have to recruit their own volunteers. We had to report on the names of each volunteer and have them sign a liability waiver.
The main gist is that you should have as many cups of water already poured as possible. If the race is big, it sucks when you can't pour fast enough to keep up with the runners.
And there are the logistics of getting the tables to the site, getting the fluids to the site, making sure you have everything you need on both sides in case you can't cross the street int he throng of people.
I suspect most marathons have written protocols. You may want to contact an RD from a race where you liked the logistics and ask for info.
Ask your doctor if getting off your ass is right for you.