My understanding of the rules (both inland and international) is that the term vessel includes any vessel used for transportation on the water including row boats. The rules are the same for them as any other boat. The problem is they very likely don't know the rules. I would keep an air horn handy and use it when required. In our neck of the woods the challenge is ocean kayakers. They often cross from island to island in a line creating a huge obstacle for any other mariners. They don't realize that they have gone from being one 18 foot speed bump to a 180 foot speed bump (assuming 10 kayaks). Early and liberal use of an air horn is warranted and encouraged.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217