I think the distinction should be easy in this case. First of all, the great lakes are sweet water, not just water with low salinity. But most important of all:

I think the distinction should be easy in this case. First of all, the great lakes are sweet water, not just water with low salinity. But most important of all: they are not directly connected to the ocean. All oceans are directly connected to each other, a ship can change between them without using locks, but they are required to reach the great lakes.