When i changed the belt on our 2008 R25 ( Mase 2.5) i had to remove the back wall of the "Cave" there are 2 allen screws that hold the Genset cover on that i could find no way to remove except by taking out the aft wall. there is another Allen screw on the outboard side of the cover that must be removed as well. After removing the cover it is pretty straight forward, the belt tensioner to lossen it up is part of the bracket that holds the Sea water pump in place. if you look from the side you will see the arraingement. I don't remember exactly the size of the fasteners but it is pretty obvious once you get the cover off.
Hope this helps
Rich