Fishermen around the coasts leave strings of pots or nets on the seabed attached to floats at each end. These floats usually take the form of a bigger object which is attached to the underwater gear and a short line to a smaller float which is used to retrieve the gear.
What causes problems is that the floats can be very inconspicuous. I have seen a single transparent plastic container/can used as a float. I have seen floats pulled nearly under water by current. And, worst of all in my opinion, at slack water a considerable length of floating line can spread out on the surface nearby.
Getting line caught around stern gear (including propeller) can cause boats to become disabled and in need of help. The fishermen themselves avoid this usually by fitting rope cutters to their boats. In my experience, rope cutters do not always work: line can become caught around a keel or skeg also.
An extra rant: when gear (pots etc) get wedged on the seabed, fishermen sometimes just abandon the pots, line and float. These abandoned floats get covered in slime and marine growth, making them even harder to see. In one case that I encountered, the abandoned float had got wrapped around some obstacle so that it was just below the surface.
So what to do: if everyone used clearly visible floats (such as those fitted with a flag) in contrasting colour (not white/transparent) and used weighted line from the float to the pots/nets then the problem would be minimised.