Oddly enough this cracked me up.. for this reason... most people that have freshwater tanks will put these types of things in their tanks and not think twice. Shipwrecks, sunken ruins etc. reality is naturally though most of those things would occur in saltwater more than freshwater. Not very many sunken pirate ships in lakes :lol:
It is a personal preference. There should be no reason to worry about insulting someone. They gave you a gift with good intentions. Doesn't mean that the expectation should go any further. If you use it, cool, if not should be cool as well.. otherwise the original premise of giving a gift is lost. I think everyone puts entirely too much emphasis on what others think when it comes to this time of year and gift giving and trying not to "insult" one another.
If you really don't want to use it, don't. If you want to see what it will look like covered in coraline, put it in your sump/fuge-- wait it out and then decide. Or just do what I know many do... plunk it in the tank when he is coming over... lol j/k.
Funny enough when I was opening it I felt like the kid for "A Christmas Story" wanting a red rider bb gun but getting the pink bunny outfit.