I don't know what the fuel plumbing situation is on your boat, but I can tell you this: if the valves are not well-labeled, sooner or later, someone is going to operate the wrong valve. That could be you, in the dark, in a hurry, in a crisis. Of course, Murphy's law says that when the error is made, it will be at a time to have maximum consequences.
Here's a cheap and easy way to clearly label valves:
- Obtain some white (or light colored) shrink tubing of appropriate size
- Write the label text on a suitable length of the tubing with a sharpie (the permanent kind)
- Shrink it onto the valve handle. The text will shrink along with the tubing, and become darker. It won't rub off.
Of course, this won't prevent incorrect valve operation, but it will make it less likely.