I'd expect DNA's half life would depend on the environmental conditions. The conditions on the surface of the earth and at the bottom of the ocean are vastly different.

DNA stabilizing:

Much lower T (below 0C) Practically no radiation (an ocean above you blocking it all)

DNA de-stabilizing: Corrosive chemicals spewing from a nearby volcano (but only if one is near you!) Radiation from said volcano and rocks (but they'd have to be closer still, due to shielding from the water)

So unless the 500 year half life is the "self-damaging" rate, I'd expect DNA at the bottom to last much longer.