I'm not sure how much of this community actually use a Mac to play, but if you do, and have updated to macOS Sierra ^(if not go to the app store), then it's really handy being able to have youtube videos and tutorials on the screen rather then in a separate window.To activate, make sure your minecraft is fullscreen by clicking the green circle, rather than by the game's video settings.For a youtube video, simply right click on it, then right click again to get apple's right click menu. Select "Enter Picture in Picture". You can then resize and move the video to any corner of the screen. I find top left works best. Swap back to your minecraft window and there you go! It's great for tutorials as you can pause whenever you want. You can also drag it off screen to hide it.Here's an example of what it looks like: http://imgur.com/a/OOyNGSorry, this is not a dam.
Make a surrounding structure, and start filling in the water with gravel or sand from the surface. Make complete rows: like this. Then dig underneath, put torches 1 row at a time and dig up until it falls: Like I'm about to do here. Just don't release a row without having another row behind it to hold the wall of water. Then you can collect the sand/gravel and make the next row, so you need a minimum of 2 rows at maximum width that you can reuse easily. This also uses no tools.