[Destroyer]

I could just say it's very important, but that would not be me and you would not understand why.. so...

It's pretty much like any other screw / nut unit. Thread seal does what the name says... it seals the gap between the threads and the nuts (typically 25% of the size of the hole). The threads on the bolt are made slightly smaller than the threads in the hole...that way they can screw together without binding. But that gap between the bolt and the hole also allows air and liquids (can you say... gasoline?) to travel the length of the thread. So when the gas starts sloshing around in the tank while on the water, or if it's a really hot day and the gas in the tank gets heated and starts to expand.... guess where it goes?... That's right...it goes to the nearest opening. Usually. as the name implies, that's the breather tube, but if there's an opening below that opening, like, say...the unsealed threads on a sending unit......well, you get the picture.

Judging from your question I'm thinking that you forgot to seal the threads before you installed the tank, and now it's gonna be a chore to get to them to seal them.... but think of how much more of a hassle it would be to have to swim back to shore because your boat decided to explode. Do the right thing...

Or do like Macojoe and use a gasket. The gasket sits between the tank and the sending unit flange and, when the screws are tightened, pinches on the threads of the screw, making a seal. So....gasket or sealant.. but you need one or the other.