it doesn't look like it's waterproof to any degree. Even the connections don't look like they would stand up to it for very long at the sensor end. i have used a Lowrance (LMF200?) for this in the same situation as you. The nevman unit was tuffed in a boat with no inbuilt sender. I bought the lowrance unit and sender, which is designed for marine use, but it won't be as cheap as that. The only downsides to such units are that they rely on 

1. Fuel flow being accurate
2.  The user remembering to manually add the fuel to the menu

Try as I might, I could not calibrate it to any better than 10% out. Fortunately, the 10% eror was positive, ie, it would think it was using 10% more than it actually was, so you always had more fuel in the tank than you thought. I dealt with this by draining the tank twice a year when I was using this a lot. Not as hard as it sounds, on that boat I just pulled both breathers off, stuffed a bolt up one to block it, and introduced gentle air pressure into the other one to get it flowing, ran it into jerries. When empty, put the fuel back in and tell it how much, so you were starting from scratch. May not be as easy on some arrangements.

Also,it will not account for you being siphoned without your knowledge. My new boat came with a sender fitted to the tank, and I didn't  buy a gauge and connect it first off, relying on the fuel management from the motor. 

But I did it later, just as a double check.  A gauge and sender are not expensive to buy, probably less than the Lowrance setup I mentioned earlier, but it will cost to have it fitted. The fabricator will need to remove the tank and purge it before even thinking of welding. You don't happen to have a later outboard with an ECM, where you can monitor fuel flows and levels via the netwrk?--probably not, I'm guessing.

 No the outboard I have is a 96 97 ocean pro evinrude, but thanks for the reply I might give this unit a go for the price I don't have a lot to lose, definitely goin to be cheaper than getting a fabricator to fit a sender unit just need something to give me some kind of idea as to what my range is.

  The easiest and best "no guesss work" method is to dip your tank but unfortunately few small boat tanks are set up for it. Ive found, after owning several boats, that filling the tank and checking distance done against a refill is pretty good, particularly if you do it over a few trips with different conditions and boat configurations (for example carrying divers or lots of gear, trolling or crook weather etc). Round up your consumption to the next highest value for a reserve and you're done. Don't get tempted to be optimistic!!

  Ranmar, we fitted one of the Lowrance NMEA flow sensors to run through our GPS. It worked ok although we never tuned it particularly well. We've disconnected it for now as we found after a huge ammount of  !)*&^%()?> around it was causing the GPS drop out. Never figured out why so we've reverted to the above method.