Lots of answers already, but one important thing that i would like to add;
Use the flattening/equalizing option of the Hoe first on any ground you intend to raise, because the game counts maxium hight from the lowest block of natural genarated terrain.

If you plan to raise the ground high up, or have locations that are under water that need to be as high as the island, you will want to make sure to raise the generated terrain as high as posible first. As you can only raise the last/highest ground block in a location.
This can sometimes be a difference of more than half a meter, and any ground you raise after that is higher because of it.

It also helps with flattening the surface a little, to make sure you have a decent flat ground to build on.