error: Could not load file or assembly 'Interop.SHDocVw, Version=18.104.22.168, Culture=neutral, PublicKeyToken=db7cfd3acb5ad44e' or one of its dependencies. The system cannot find the file specified.
This was particularly annoyng since I had installed WatiN through NuGet.
Turns out this is a known issue tied to the way WatiN.Core is signed. To move past this problem, select Interop.SHDocVw on the references folder of your project and set the “Embed Interop Types” property to False.