Why didn't Dumbledore bring backup? (Harry doesn't count)

[Michael]

No: HPFGUIDX 149196

Going to a secluded cave to retrieve/destroy one of Voldemort's
horcruxes seems to be a pretty dangerous task, even for Dumbledore.
Why didn't he consider bringing another OP member along? (maybe
Snape?) Even if this person could not fit on the boat, it would have
been the safe thing to do. Why wasn't Harry surprised or concerned
that he would be acting as Dumbledore's backup?

Michael.