Why are the people angry at Kit on the boat?

The ship on which Kit's bound for America makes a stop at Saybrook in Connecticut to unload passengers and pick up provisions. After exploring the less than impressive dock for a while, Kit gets back on the boat with the other passengers to head on back to the ship. On the short boat ride, a little girl accidentally loses her doll in the water. No one seems ready to help the poor young lady retrieve her toy, which makes Kit quite angry. So she jumps into the water to try and fetch the doll herself. Kit's closely followed by Nat, worried that Kit might get into difficulties. Thankfully, though, Kit's ok; she swims safely back to the boat, dolly in hand.
Kit's rather pleased with herself for her impulsive act of bravery. But the other people on the boat are far from happy. As well as thinking Kit's actions reckless and irresponsible, they're also rather suspicious of a girl being able to swim. Apparently, this sort of thing is rare in New England. This is just one of many culture shocks that Kit will encounter during her American sojourn.