There is a "Fraser River Debris Trap" near Chilliwack, I believe, that tries to collect debris floating down the Fraser (a very large river) before it can get out to Georgia Strait. It's been funded by a collection of government and corporate money and each year the funding is threatened to be cut off.
If that ever happens we'll be much worse off.
Another source of these keel/prop/rudder killers is when log booms are towed in marginal weather.. booms occasionally break up or lose a section.. leaving full sized logs drifting about, in clusters until they become separated by wind and current. They are difficult to spot in chop, or poor weather. On days like that I can't imagine driving a SeaRay