I suspect you probably have a fuel tank filler cap that resembles this one:
http://www.tanksinc.com/index.cfm/page/ptype=product/product_id=475/category_id=166/mode=prod/prd475.htmI am certain that when the 2001 Boston Whaler boat was delivered, there was a wrench or key for that filler cap included. Check around the boat for one. Perhaps it is stored somewhere.
When you find it, the first thing you need to do is attach a float to it. Those filler keys are very prone to falling into the water. For a float, you need to get several of these:
http://www.whalerapparel.com/product-p/bw1128.htmPut your boat ignition keys on a Whaler float, and your fuel filler cap wrench on another, and maybe any keys for boat storage cabinets or trailer locks on a third float.
Also, check the status of the retainer for the filler cap. Usually they are retained by a small chain or a small diameter aircraft cable. However, the chain may be worn. Use care when removing the filler cap.
On my 1990 Boston Whaler the filler cap chain is now loose. I actually like it that way. As soon as I unthread the cap I put it back inside the boat. The thought of the filler cap loose and dangling over the water on a 25-year-old little chain makes me nervous.
Also--make duplicate keys for the boat ignition and any cabinet door locks. Keep them somewhere on the boat in an inconspicuous place, maybe way down at the bottom of a locker (that you don't keep locked) under a bunch of other stuff.
Also--the dock attendant at most fuel docks will have a wrench for that filler cap, but you will need one at the highway fuel station. That is why I generally keep the boat keys in the truck glove box. When buying fuel at a highway fuel station, I don't have to get on the boat and look for the filler cap key.