sorry no mention of the Vincent surname in Youghall
It looks like a waterman is a similar occupation to a boatman - so possibly William worked with this person :
Custom House, Strand section
Tide Wailer(*) & Boatman - George Sloane, Mouse street.
(* - letter is smudged - I'm not sure if thats an L or a T )
The nearest Vincent surname from Slaters directory 1846 is :
N. & R. Vincent attorneys, 8 South Mall, Cork (city)
Shane