Use block hints to specify water surface height

This supersedes #1465 by simply using a block hint to specify the water level. I'm surprised that is was to easy to fix.

A screenshot from PolyWorld illustrates the improvement:
![image](https://cloud.githubusercontent.com/assets/1820007/5587023/e891c1a4-90df-11e4-8e8b-96f0c0fdb379.png)

Kudos go to @MarcinSc for the idea of using block hints.

Closes #1371
Closes #1466