it always ends up being an ocean square ..

go into terrains then select coast then go to a tile where you want your lake and left click (make sure it has land round it) as you will see that tile is now a lake. click on tiles next to it to make it bigger

it always ends up being an ocean square ..

Worldbuilder calls it an ocean, but when you drop back into the game, it turns into a lake. If your lake is more than 10 squares large, it also ends up being an ocean.