The threaded flange should provide all the waterproofing you need on the pressure side. If not add some thread tape and re-tighten. The gasket is there to stop the 1500L or so of unpressurized water inside the spa leaking back into the lining.Think of the silicon as loc-tite - it is there to stop the flange working itself loose.

The threaded flange should provide all the waterproofing you need on the pressure side.


 Thought about that..The threads are rather loose fit to say the least


The gasket is there to stop the 1500L or so of unpressurized water inside the spa leaking back into the lining.

What water proofing? No lining, just a solid polypropylene or ethylene spa wall

Lets say there is no seal /gasket on the inside.. the water 'leaks between the flange and spa wall...cant go anywhere, cant leak.
IF the seal/ gasket on the outside is secure, then any leak thru threads or the inside flange doesnt matter, they cant get out.

what does also make a difference is that at the time of install in the factory they hold the flange and gasket still and turn the jet body to tighten it down. That way you are not turning against the friction of the rubber gasket.

I sort of guessed something like  that would be the case.
I see no reason why they could not do it the other way round thu..gasket between jet body on the outside, and turn the flange.
Which would be the same as repair a jet leak after jet installed.???

At the end of the day, any leak is only relevant if it leaks to the outside of the spa...and the only place that can happen is between jet body and spa wall... not in the inside wall, that stops nothing

BTW, another reason for having the flange static and the body (outside of spa) turned is no tools inside the bath = no scratches 😉

OK this is what has happened and done.
1/Dug a little more foam away so can clean up jet bodies easier and not 'load up ' hoses connections while doing so

 Guess what? !!!!
 That flat surface of the jet body that faces onto the spa wall has a O ring groove in it.. and it matches exactly the biggest O ring in my boxed supply of neoprene O rings.

It seem therefore the gasket does go between the inside of the jet fitting and spa wall..not as a seal but more likely a spacer and make removal easier by not clamping up on its self over time ....

As to sealing the threads, cant see any reason why to.

So I have simply smeared a little  silka marine urethane onto the clean spa jet body groove, pressed the nice fitting O ring in so holds in place.
 A smear of urethane on the cleaned outside spa wall.
 
Set the inner jets and locking ring (not shown on pic)
 screwed the jet flange in...leaving the jet face off for now.
 The jet face pushs in easy and couple expanding clips hold it in place.
Will leave these off till know no leaks in a few days... just makes easier to remove jet bodies with off.

Cant believe that at install they remove O rings and simply fill up everything with silcone???
Either the design of o rings is faulty .. and since these have been manufactured for many decades in the 'states, used all around the world.. that I doubt.

What I recon is initially they have been installed, and cranked up real tight because the person has not been instructed  that to do so, will deform/ roll the O ring and will leak...
Then drawn up their own instructions and inaccurate  pic (above) and that been the practice for the next few decades..

Appreciate all the comments above guys...
I was doubting my non professional  logic over their 'professional' install instructions.

Thanks
Steps

Yep next time you are passing thru with a nice cray m8.

That last one and the snap was greatly appreciated when I was crook.