Hi fellows, I'm Danny Digger, proud owner of this peace of land...
Let's build this lake together, I love to swim in natural water without chemicals :-) Impossible in Sims2? No, not at all! I'll show you how to make it, please follow me! ----->

Looks boring, I can't wait to get started!

First of all, even out your land if it's not even already, using the tool shown in the picture!

Then you should mark the borders of your lake, in order not to lose track...

Now you can either highten the terrain around your lake or dig out the lake itself; I prefer a combination of both, first I start digging by 4 clicks...

4 corners digged down...

Then I start in that hole and drag the new hight to the rest of the land inside my borders.

done...

Now we'll make nice gangplanks. Chose the foundation shown in the next picture. It's very important to start at the "land"-side of the lake - otherwise the foundation would be same hight with the land!

That's it for now...

You should know the trick: Sims start swimming as soon as they jump from the dive or climb down a pool-ladder! They would swim everywhere and never stop, even inside the house and on all floors. They only jump up if they shall do something, like cooking or sitting down. So you have to give them a limit by hight-differences between the land and lake of 5 clicks difference minimum! We already digged down 4 clicks and now I start to highten the borders. You can do it any way to your liking, but 1 more click minimum.

around all the lake...

done... now - to make it more realistic - I highten some areas inside by only 1 click (but not in front of the gang, and not more than 1 click!)

tataaaaa, here comes the water! It's texture...

Chose 2 or more terrain-paints water to make it look like a real lake; I chose a darker texture in the middle, where the water in nature is deeper...

I'd recommend to use the cheat "move_objects on" for decorating, this way you can place plants nicely wherever you want. Also use "boolprop snapobjectstogrid false", so that you can ignore the grid. That gives you better possibilities to plant where you like to... Remember to shut down the latter cheat again after you're finished, by typing "boolprop snapobjectstogrid true"!

Nice, isn't it? I used a stone-texture for the borders, that makes it look natural. But you can also build a seaside-look by using sand... it's up to your phantasy :-)

Now add stairs...

and railings if you like...

We still have the "move_objects on" cheat on, right? This cheat enables you to place the dive and pool-ladder on your gangway! Shall I try it?

Ready, steady, go!

Awwww what a nice swim!

Very important: To test if your sims won't swim forever and you didn't make any mistake in hight-differences between land and water, make them do something outside the lake (let them go to any point outside e.g.). As soon as your test-sim uses the ladder to climb out of the "water", you won!

Now I'm starving! Thanks for reading and good luck with your own swimmable lakes / ponds / seaside lodges!
PS.: You can download my first uploaded lot with swimmable lake at my simpage, it's called "The S(w)imside Cottage"; it features 3 BR, 2 baths, singletile-stealthstairs, everything for your sim's comfort. Including a sandbox in the garden with toys and little boats in the lake for your sim-kid's fun :-)