The crossing from the mainland can be rough, but on a good day, a gentle rhythm keeps pace among the islands out at sea. ‘‘Jump to an island, and stop. Jump to an island, and stop,’’ said Ken Taylor, of Plum Island Kayak, which charters a yacht to carry customers and kayaks out to the islands for a day. Conditions permitting, more adventurous paddlers can leave the relative calm of Smuttynose and Appledore islands to join the seals near Duck Island. ‘‘We paddle around and say, ‘Wow!’’’ Taylor said. Plum Island Kayak, 38R Merrimack St., Unit 101B, Newburyport, 978-462-5510, www.plumislandkayak.com.