Sergei Bobrovsky made 18 saves for his second shutout of the season and 36th in the NHL.
"He had the toughest job in the world tonight," Florida coach Andrew Brunette said. "When you get no shots and then it's a breakaway and it's another breakaway, to stay mentally focused, for me, I thought that was his greatest asset."
Anton Forsberg made 45 saves for the Senators (19-29-5), who have lost three straight. Ottawa's 18 shots were its fewest this season.
"[Forsberg] was outstanding for us. He kept us in it," Senators forward Brady Tkachuk said. "It stings. It [stinks]. It's definitely frustrating, for sure."
The Panthers were 2-for-9 on the power play. The Senators were 0-for-3.
"You've got to be more ready than that to play," Ottawa coach D.J. Smith said. "We knew that they were going to come out hard. The first few shifts were all right, we were hanging in there, coming with the push. And then the penalties start and it's just downhill from there."
Marchment gave Florida a 1-0 lead on the power play at 10:04 of the first period when he crashed the net and tapped in a loose puck. The Senators challenged for goalie interference, but the call on the ice was upheld by video review.
"They were just hungrier than we were," Smith said. "They beat us to all the pucks."
Ottawa was outshot 25-7 in the first, two shots shy of its record for most shots against in a period (27 in the first period against the Montreal Canadiens, Dec. 12, 2015).
"I thought the fortitude and the resiliency tonight was a really good test for us," Brunette said. "It wasn't coming easy. We had so many different looks where pucks have gone in in the past. Lately, they haven't been going in, but we stayed within the game plan, within the structure and how we wanted to play. We didn't deviate. We didn't cheat to make it better."