The "higher level" areas roughly radiate outward from the altar, the farther you go out the harder it gets. Also depends on your seed. One time my starter "island" was a continent that took up 1/6 of the map, sometimes you'll have a very small island (like i did on purpose one time). Also save your greydwarf eyes, and treasure these nocturnal battles. You will need many, and if you use them right and think a bit getting your dead bodies will be very easy. Dont ever make the food that uses eyes as an ingredient.
And yea rafts are for big rivers. dont go ocean sailing without the bronze age boat