I don't have access to any manuals at the moment, but I'm fairly sure that what you are referring to is the destination, not the departure point. If no alternate is available for the destination, then you are correct in saying offshore alternates can be used (but not for payload enhancement), a bit like island holding fuel for FW a/c. However, the departure also must have an alternate when IFR, and for twin engined a/c it should be 60 min still air on one engine (90 min on two if 3 engined). Normally this would be the departure airfield. Then as above....
Can't quote any reference numbers at the moment, sorry.
------------------
Another day in paradise