I applaud your actions ! It's bad enough to have to break off a length of line, when hung up .... but to blatantly throw a whole spool of line into the water is just not the actions of a "concerned sportsman".
When at home, changing line, I keep my old line until I get a good amount together - then take it to my local tackle shop, where it is returned for recycling. When on the water, I wrap the line around my hand and tie it together - then place it in a bag for disposal.
Checking behind the prop is good advice ..... but, also check behind the trolling motor prop !! With trolling motors costing several hundreds of dollars, you don't want to neglect checking them, as well. ............cp