Sorry, I had to leave but I didn't have time to copy it into a text file.
Here's the complete version:
XXXXXXXXX.
XXX_XXXXX.
XXX%X|++X.
+>X%XXX>X.
XXX~XXXXX.
XXX~XXXXX.
XXX~~~~~~~
XXXXXXXXX.
XXXXXXXXX.
level+1
XXXXXXXXX~
XXX~X_XXX~
XoX%X%XoX~
X<X%X%X<X~
XoX_X_XoX~
XXXXXXXXX~
F#+++#++F~
F#+++#++F~
XXXXXXXXX~
level 0
XXXXXXXXX~
XXXXX~~~~~
XXXXXXXXX~
XXXXXXXXX~
XXX+X+XXX~
XXX+X+XXX~
X+++X+XXX~
X+XXX+XXX~
XXXXXXXXX~
level-1
[/font]
X is just wall (or anything that blocks water, really) because I didn't bother to go find the real wall tiles.
+ is floor tiles
> and < are stairs connected to each other. Ignore the sign and use common sense to figure out which way they go. Up/down stairs can be used at the higher levels.
. are open spaces above the body of water. The pump stack must get to at least the top layer with water in it so there's somewhere to put the water.
%
% are pump tiles;
Z 0:The left one faces north, the right pump faces south.
Z+1: it faces south.
The left pump is actually the bottom of a pump stack, which can put the water wherever you like (this version assumes pumping it back into the lake or ocean)
| is a vertical axle, which leads to a gear assembly hooked to one of the levers. Do try to remember to put the axle over the impassible part of the pump. Otherwise, water will probably rush up from the lake.
~ is water;
Over to the right is the body of water you're making an airlock in to/out of.
To the left is the fortress (on level+1 there is a path leading to it.
# are grates with channels under them
_ are channels
o are levers;
The left two levers drain the airlock and open the left two floodgates/doors.
The right two levers fill the airlock with water and open the right two floodgates/doors.
You could also put the levers inside the flooding chamber if you like, though I would recommend having the draining lever in your fort, but you don't have to if you like having fun.
F are floodgates/doors, controlled by the levers.
You can always add more layers to the pump stack, and you can also probably figure out a way to power the pumps from below.
Also note that the right pump at Z-0 will probably flood over its passable tile, so plan for that by using a door rather than just leaving a corridor.
Place fortifications where you like at the in/out water paths to prevent nasties from getting to your pumps.
EDIT: Now I want to go and find a site where I could actually use this =/
The way it should work:
Entering from lake:
You swim into the open chamber and wait (or pull a lever or two). The opening behind you shuts, and you hear rushing water. The water level slowly drops away as it is pumped out through some nearby grates. Once the chamber is dry, (you pull another lever or rig something with a pressure pad) an opening in front of you appears, and you begin to explore the underwater fortress.
Exiting to the lake:
You enter the chamber again, richer than you were when you entered. You pull a lever, the doors slam shut, and water starts bubbling up from grates in the middle of the room. When the room is full of water, the gates leading outside open up and you swim out, only to be eaten by a school of carp.