We have an annual Ohio River Sweep day w/all volunteers (6/16/09 this year) that gets a fairly good turnout. And there are lots of these sort of efforts all over the country.
My dad taught us kids (9 of us) to pick up the crap others dumped behind and other hazards. AND He practiced what he preached.
I used to get home from school and after dumping my books, I'd jump in our rowboat or go sailing in my snark or dad's home-built daysailer -- if I personally saw stuff floating by, I'd snag it, toss it in the bottom of the boat (if I could handle it size-wise) or tie my painter to it (eg. logs, etc.) and make a quick trip to shore to beach it or when I was done having fun, toss it in our garbage cans before heading into the house for dinner.
I do this in parking lots today as well (pick up somebody else's trash and toss it in the appropriate receptical). It really agravates me to watch people liter by land or sea.
It would be nice if people didn't liter and if people took a few minutes to redirect the trash they come across along their paths.