Floating Prop Wrench
Prop Wrench and transom plug wrench combo.
OK, no big deal- Until you need one!
Let's go through the paces here. It will undo your V6, inline 4, or outdrive prop when you're stuck and hanging over the side doing a swap. It floats (do I need to tell you why that's a good idea?)
It's Yellow, so it should show up in the bottom of the spares bin, and it has a transom plug wrench on the other end. It won't rust up when you store it, and it's very inexpensive. So, keep it with your spare prop and nuts, unless you haven't bought those yet.