Sea State 1, I am told by the splendid chap who looks after the AOC paperwork. But then he also swears that he has seen evidence that an R44, once landed on water, can depart if they do so quickly before the belly-pan floods. Personally, I have no intention of trying that particular manouver.
More of a problem for the R44 intending over-water passenger transport is the lack of a second engine. That means heliroutes along the Thames, Battersea, etc are all fine. Unfortunately, no commercial flights to France due to the width of the English Channel.