Beauvillier gave New York a 3-2 lead when he stole the puck from forward Trent Frederic in front of the Bruins net and scored on a backhand. It was Beauvillier's first goal of the season in his 10th game.
"That's nice, I love seeing him smile," Islanders forward Mathew Barzal said. "He loves scoring, so whenever he gets one or a guy like that who hasn't had one yet and he gets one, that's definitely a big boost for the bench. Everyone loves seeing him score."
Eberle made it 4-2 when he finished a 2-on-1 with Barzal at 10:45.
Pageau scored on a shorthanded breakaway at 12:24 to make it 5-2. It was Pageau's second shorthanded goal against the Bruins, and of his eight goals overall, four have come against Boston.
Lee then made it 6-2 at 14:44 with his Islanders-leading ninth of the season, and Wahlstrom scored his second NHL goal on a rebound at 16:33 for the 7-2 final.
"There wasn't any [urgency in the third period], so that's a problem. I don't usually see that with our club," Cassidy said. "We've got some young players in the lineup, so we're going to live with certain mistakes. … It's a good defensive team; they don't give you much. You need to have patience.
"We found our game halfway through the first, into the second. We're in a good position in the third, and we made some poor decisions."
Ritchie gave Boston a 1-0 lead at 1:02 of the first period on a redirection of Jakub Zboril's pass from the right circle.
Adam Pelech scored his first goal of the season on a rebound near the left post to tie it 1-1 at 3:16.