Also, if this line was plugged, how would you know if the tank is leaking? It's a sealed compartment.
You will smell it
I don't suggest you to put a plug in there, if there is not then GW had a reason to not install one.
When i pulled the fuel tank on my RIB fuel tank was in a gelcoatet compartment and there was a drain hole to the bilge on the end too.
If you have whatever leak (including fuel) it would fill a sealed fuel tank compartment and the fuel tank would start to float sooner or later. Also a fuel tank sitting in water don't last very long, so a drain hole is needed to drain any liquid from the fuel tank compartment (aka fuel tank coffin). Thats also why the fuel tank has to sit on PVC strips, to allow water to drain and then dry under the fuel tank.
There are some boats with glassed in fuel tanks and in theory they should sit in a watertight compartment what should make fuel tanks living much longer. However this system does not always work as supposed and if the fuel tank has y leak then owner has to cut the deck to replace the fuel tank.
My Venture 34 had glassed in fuel tanks installed in 99 and there was no lea, but there was a limber hole, but in my case dry.
Chris