Hi!
I'm Bob The Builder, and this time i'm gonna show how the build under water.
Wouldn't it be cool to build a house like this.
You can even use an underwater house just like any normal house!
Are you ready?

First let's make some foundation, it doesn't have to be exactly like shown.
The only important matter here is the distance to the sidewalk.
(we will need room for a connecting stair)

paint the foundation(choose a paint that will show you where each new floor will begin)

Now choose the biggest water-tool like this.

press until you are aproximately one floor down.
(i chose a paint that shows it with a white border)

Well we want to go a bit deeper, so let's press a bit more...

Well this looks like a fine depth for our house.

We need to expand our lake.
Don't make it as deep as it is at the foundation.

Select the foundation-tool and start at the deep end close to the other foundation(at least one tile away from the other foundation, or it will level out with the foundation above water)

It should now look something like this.

Select 'connecting stairs' tool and start above water and down to the foundation under the water.
(If the stair won't connect, you're probaply at a wrong depth, undo until you reach the point where we selected the depth and try another depth)

Make some more foundation at the bottom like shown.

And also on the other side.

Now we have a base foundation for our underwater house.

Before we go any further under the water, let's level the ground in front of the upper foundation like shown.

place a connecting stair like shown(or we won't be able to get to the underwater stairs)

Let's build a small test house under the water.

place a door in the house.

And Bob has gone into our own little atlantis.
The rest is up to you now, it's like building a normal house once we've made an underwater foundation.
You can select any depth that you want, but be aware that the deeper you go, the harder it can be to place things and build.
If you're having trouble building and placing things under the water, try zooming, rotating the camera until you find a suitable angle.

That's it, enjoy and experiment with you're new knowledge

Best Regards
Frillen