Last Saturday, while returning home from a nearby port (35nm to the north)
on Lake Michigan this happened to us when we were about 8nm from our port.
I'll report the incident and let you all make your own judgments.

It was a sunny day, 10kts. from the west, and we were making roughly 6kts.
southbound, my wife at the helm. I'd just lain down on one of the cockpit
seats when I heard the rumble of a couple of large gas engines throttling
down. I peeked over the coaming to discover a go fast boat, approaching
from the rear, had slowed and nearly stopped about 30 yards off our
starboard beam. I grabbed the binocs and took a look. The skipper was a
shaggy-looking middle-aged man dressed in a muscle shirt and his three
passengers were all younger men dressed in heavy, dark clothing--sweaters,
dark pants, etc. Not exactly what one would expect for an afternoon outing
on the old go-fast. All were standing in their boat, the Panama Jeanne,
about 38' long. They turned toward us and approached at slow speed, came up
behind us, and stalked us for about 5 minutes. Then they pulled up along
the port side. During this time I went below and obtained our 25mm flare
gun, loaded it, and stuck two additional flares in my pocket. A lot of good
it would have done, but it made me feel that I was at least doing something.
The skipper then yelled something to us. We asked him to repeat and he
inquired as to the distance and direction to our home port. We told him 6
miles (a lie, but we could see the breakwalls of the channel in the
distance). He then conferred with the other young men, and finally said,
"You have a beautiful boat." I said, "Thanks." After stalking us for a
while longer they finally drifted off to port (toward shore), idled along
for another five minutes, then sped off to the south.

The experience was unnerving, to say the least. It certainly makes one feel
vulnerable when traveling at 6kts. My take on this is that they thought a
single woman was traveling alone, and when they saw me they decided to check
things out further before reacting. It is possible they might have thought
me armed, as my arm and hand were extended into the cabin, holding the flare
gun, while I was standing in the cockpit.

OTOH it could have been innocent, but such behavior is certainly suspicious.

What's your take?

Max