How do I know if water is fresh or salt?

In New World there are different fish and bait for each water type. That said, it's somewhat hard to tell what water type is which. Obviously a beach is saltwater and a landlocked lake is freshwater, but It's a bit harder to tell when the areas mix.

For example, there's a bay/lake/gulf/river thing near First light that surrounds the Fort Island. Is that fresh or salt?

Salt water is denoted by the black lines the are around the coast as seen on the map, while fresh water does not have these lines:

The wiki page for salt water some-what says this:

It is usually a black color on the world map, but not always

There are some exceptions however, such as the swampy areas such as Cutlass Keys and Weaver's Fen, where that water is considered salty.

If you are at the actual body of water, you can't collect salt water, whereas with fresh water you can.