Mute Swan (Cygnus olor)
These swans were spotted swimming on a fresh water lake (Lake Velence), the water temperature at the time was around 25C. The lake is abundant in reeds and frogs, and I saw people fishing so I assume fish too (despite the fact that none of the pictures show any of that!). Sandy lake bottom.
Despite hissing when I got within in 1.5 meters from them they don't seem to mind people too much as they swam right into the middle of the most densely human occupied areas of the lake. I'm adding a link to a site with info about the lake too, just incase someone finds it interesting.