Does the Coast Guard patrol the lake to chase off stray sailors?
Not sure what you mean, but Newport is on Lake Memphremagog which straddles the US / Canada border. So yes, the lake is patrolled by the Border Patrol to keep out smuggled maple syrup and other undesired items.
I was thinking more about US boaters, fishers, sailboats that must try to figure out the border in the middle of the lake perhaps in a storm. Or even skaters in winter.
They have rules requiring boaters to check in at specific border docks, like at the Newport municipal dock. Plans can be filed online ahead of time, but they must still stop at a port of entry. More details
for Lake Memphremagog.
I can imagine in older times going out on the water. Let’s go for a spin around the island seems like a natural day trip. And at a certain time, stop for a case or two of liquid refreshment might be in order.
Lots of stories about Lake Memphremagog (and Lake Champlain) being highways for liquid refreshment during Prohibition.