Hi David,
Thank you for filling me in (very punny) on what you're doing and what the problem is.
All right, so here's another great way to do this.
Make a feature class called Lake Mead.
Make another called Islands.
If you've got Lake Mead and the Islands all on one feature class, export the selection of all islands to a file called "Islands" on your project geodatabase. The same goes for the Lake Mead Polygon.
Use Erase tool to remove Islands from Lake Mead.
Adjust line width of Lake Mead Erase.
This always works. Let me know if this solution works for your situation.