Please help with my custom map

Hello im making a custom map of the Napoleonic era. and i had something that was kind of frustrating me. i was wondering if someone can please help if they know the solution.



as seen in the image there is a lake in Sweden indicated by arrows. does anyone know how to make the lake defined as a body of water instead of having the same color of the country (the map thinks it land) all suggestions are well appreciated.

Step 1: Draw your lake

Step 2: Draw Sweden

Step 3: Copy Sweden and the lake

Step 4: Paste

Step 5: Move to position

Step 6: Delete old borders