I do not believe that the boats-men or fishermen calling themselves 'SAHNI' belong to same creed of Sahnis from Punjab. Literally those from Bihar or eastern Uttar Pradesh were 'sa hh nee' which is literally means 'who bears the burden' since most of them had boats for earning livelihood.
I other case, the Sahni of Punjab are viewed as 'Doabi Purohits'. A respected cast of Brahmins who turned as warriors. Trilochan Sahni in tenth centuries was a warrior who won hearts of half of South Indians owing to his bravery.