Yeah
It's pretty easy, make a lake with cliffs (use dirt instead of stone if possible).
Next step select the "Ramp" tool which should be right below the raise and lower tools. Now click and drag up and down between the lower and higher sections.
NOTE: Ramp will only make a ramp 1 floor down, so you can't go from land to deep sea in one ramp, you'll have to make 2 ramps, or leave a cliff.
You may realize some corners can't be made into a ramp. To fix this, raise and lower land to make the cliff smoother (Teeth like cliffs usually can't be ramped, because you need 2 squares of clearance for the ramp to work).
That's about it.
NOTE: if you don't use ramp tools like I said, and only use raise height tools, the water will not flow very well into the land, and you will not be able to walk into the water either.