This is something that a visitor mentioned at work, that boats drag increases when they hit a patch of fresh water at sea.
Thinking about this, I can only imagine that being caused by a quite specific set of tidal conditions, and the presence of a recently melted ice berg or flood run off.

Does anyone know anything more than I do about this? I'd love to know more!

there are a couple of places off the south of France where the rivers run underground and come out offshore ,last time i saw this they were trying to catch the water to pipe ashore for the locals.

I know there are fresh water "rivers" that seem to flow in certain parts of the sea. In Planet Earth they showed some underwater caves that gave the appearance of "air" at the top, but it was actually an illusion caused by the separation of fresh and sea water.

That's pretty awesome...

I remember diving into the sea somewhere off the coast of greece from a yacht and being amazed that the water didn't make my skin dry out completely. Or taste salty.

Guess that explains it.

I'd imagine its due to the density of freshwater being higher than seawater (same reason you float in the dead sea) so it's more resistance

The planet Earth bit was pretty cool to watch

Salt water has a higher density than fresh water.
So, when a boat enters fresh water, it sits lower in the water and the drag upon it very likely increases (dependant upon the keel design).

Is that not completely backwards? Salt water is denser, hence you float in the dead sea which is insanely salty.

Quite why fresh water would have more drag I have only one idea; it's less dense so the boat will float lower in the water and present a significantly higher cross-section.

After it rains, there is also, often, a layer of fresh water on top of the sea water. Shipwrecked sailors have often used the trick to keep themselves alive. Allegedly, the layer doesn't last long, but it can be a life saver.

I have heard mention of freshwater in seawater being one cause of "Bermuda Triangle" type disappearances. The roof of an undersea cave containing freshwater collapses and the freshwater is released. The lower density of the freshwater causes it to rise and to create an area of localised "low" density water in the sea. This results in the ship sinking lower into the water and literally sinking. I can't recall if this was proven or hypothesised.

Yeah, I remember something similar, but it included a release of a large air pocket under the sea bed. The air bubbles up and effectively wipes out the bouyancy of the vessel on the surface.

Sounds plausible, though aliens or timewarps are more exciting.

Typed it wrongly, meant to say lower instead of higher, yes salt water is denser

Aaaah. Was it air? That would make more sense. Fresh water could cause problems, but air would really screw things up. The only thing that makes me wonder about that is what are the chances of a cave existing under the sea for millennia containing enough air and little water.

You do get methane and other gases released during volcanic and tectonic activity, so if anything was happening on the sea bed, a release of gas could also occur. There's also a lot of decomposing organic matter on the sea floor, don't forget.