Given how common it is in the states I see no reason this couldn't be a boat-hopper. HBW say the following of four ringing recoveries involving over 1000km dispersal events:
"The most distant recoveries involved movements from Illinois SE to North Carolina, New Jersey E to Kansas, Arkansas N to Ontario and South Carolina N to Quebec. As revealed by one study covering ringing data up to 1962, those recovered at significant distances from ringing site were largely first-years; no difference in mobility between sexes. Directions of dispersal relatively random, with no predominant heading. Occurrences of individuals at locations well removed from recorded breeding range also indicate mobility; for example, one on shores of James Bay (Ontario) was 750 km N of current breeding range, while one in NE Newfoundland was more than 1200 km outside range, including not fewer than 110 km of open ocean. In Hawaiian group, race canicaudus, introduced on Kauai, Oahu and Hawaii, has subsequently colonized many other islands, apparently without further human intervention; three records on Nihoa, which would involve over-water movement of 275 km from nearest population."
In my book it's no worse a candidate for vagrancy than Lark Sparrow!