Friends say they doubt a police theory that Shannan Gilbert — missing for 19 months — drowned in an Oak Beach, LI, marsh — especially since an official search yesterday failed to produce her body near her personal items.

Cops had dug up her cellphone, purse, shoes and torn jeans nearby on Wednesday.

Her disappearance in May 2010 led police to stumble upon the graveyard of a serial killer suspected of murdering 10 people.