Why do some scientists now think that some early Americans may have arrived in the Americas via boat?

Yes, ancient welch. Yes, they have found the remains of what would have been an ancient Norse person (viking - Scandinavian - Norwegian "haplogroup was identified with remains found near "The Money Pit" here in North America. The vikings were the best at sea travel and NAV that we know of in the Ancient world besides the Romans with their Anti therum device (they lack any proof of a real expidition on sea) the polynesians were also very good boat builders, and their haplogroup merged with a little clovis people could be native american.