Lagoon missing problem below! This "How to use the lagoon tool Sketchbook tutorial should show you the basic mechanics.
If the Lagoon is missing, a problem I had, the only way I could fix it was uninstalling and reinstalling. This is less than ideal, so attempt to restart your computer, check for updates, and reopen the app before doing so.
Usually to get it back you'd go to edit, preferences, lagoon and re-enable it from there. Hope this helps