First let me say that any knowledge I have on this subject is gained as a result of dealing with rust in motorcycle gas tanks. But a metal tank is a metal tank whether it's strapped to a motorcycle or sitting inside a boat. Anything from small chains, to nuts/bolts, or BB's can be used to break the larger flakes of rust inside a tank loose. Rust can even be removed via electrolysis with a home made setup. You can find a variety of methods used and some even give how-to's, and their results. Just google "removing rust from motorcycle tank" and that should get you started. Now that you have the basis of where to start, let me cut down your search time and tell you that removal of the rust is only half the battle. The other half of the battle is keeping the rust from returning. The only successful method I have seen to do this is with a "liner". There are a few liner kits available with the 2 most popular being Kreem and POR-15. I know of a few people that reported issues with Kreem liners starting to peel or turn soft after a year or so but I also know of more that report that they used it and it worked perfect. Everyone that I know of that used POR-15 (including myself) has had good luck. I installed a liner in my motorcycle tank 5 years ago and it's just as good today as it was 4 years ago. That didn't happen without a fair amount of work and money though. The only reason I went that route was because a good replacement tank would have cost me anywhere from $200 and up if I was lucky enough to find one on ebay.
Now, having said all of that, let me recommend that you forget about that metal tank you bought and go buy a new plastic tank or a good rust free metal tank. I'm not sure what a good metal tank would cost you but a liner kit like mentioned above is going to run you $50 + or - a few, which is about the cost of a new 6 gal. plastic tank.