Okay, If I'm right there should be a "Water Level" event somewhere in the events (I think it is at Envirnoment > Scenery, but I can't check since this PC hasn't got Jazz installed)

This event has two things you can set.
1. Instant
2. Height (or something like it)

If you set instant to 1 your water will be seen right away. If you put it to 0 the water will slowly come up

Now a part that catches a lot of new people. The height should be set to the number of blocks starting from the top of your level.

In example: If your level is 64 tiles high, and you want the water to be 20 blocks high, you should set height to 44.


I hope I didn't make this to complicated for you