If I remember correctly, there was another topic asking same question: "How to make water", but I don't remember where it was.
Well, let me explain briefly how to make water:
1)
Make a pit. Draw a sector and give it a depth; 64 fracunits deep is enough. And set a texture for the cliffs.
2)
Make a control sector. Control sector is a little sector you draw outside the map. It applies effects to the sectors in the map. A little triangle is enough.
3)
Choose the water dimensions and appearance. First, set "Sector Mode" on. Now double-click the control sector.
Give it a water surface height by setting its ceiling height, -8 is good; and a water bottom height by setting its floor height, set a value deeper than lake bottom height; -96 would be good (unless you want a weird effect of water surface hovering on the lake bottom).
(
Note: I gave these heights because I know the floor height around is zero)
Give the control sector water flats (flas are floor textures). Recommendation: BWATER01.
Now click OK.
3)
Choose the water effects. First, set "Lines Mode" on. Now double-click a control sector's linedef.
In Properties Tab, click on "Select Action...". In FOF category, choose "Water, Translucent, No sides (123)". Click OK.
Back in Properties Tab, give it a Sector Tag number. Then, click on "Next unused", it'll search the next tag number after the last one you used. As it's your first time you're setting a control sector in your map, then the sector tag number would be "1".
What is the sector tag number for?
Sector tag number is a link between the control sector and the sector that will receive the effect. Then, all sectors that have a tag number "1" will receive a water effect with control sector's dimensions.
4)
Applying effects of control sector to the pit sector. "Sector Mode" on. Double-click the pit sector and give it a tag number of 1.
5)
Test your map.
More about water:
http://wiki.srb2.org/wiki/Water_Design
Now, feed me a cookie.