The German military plans to prolong its 300-troop presence in Lebanon because of a "high state of tension" on the border with Israel, fuelled in part by rocket fire from Hezbollah strongholds.
The naval forces will remain part of a UN peacekeeping team that represents "the only communication platform between Israel and Lebanon", the German government said.
The UN mission has a mandate to keep the peace along the 120km so-called Blue Line dividing Israel and southern Lebanon.
A German naval presence in place since 2006 focuses on patrolling Lebanon's coast to clamp down on the smuggling of arms and monitor sea routes to Beirut, Tripoli and Sidon.
The German military's mandate expires on June 30 but parliament gave a preliminary nod late on Wednesday to extending it for another year.
A briefing to MPs said the troops were a stabilising presence during a state of "political paralysis" in Lebanon, whose presidency has been vacant since October.
The past year has seen a "continued high state of tension on the Blue Line" including an exchange of fire in April, it said.
Israel hit back with artillery fire after rockets were launched across the border from what Germany said were "de facto Hezbollah-controlled areas" in southern Lebanon.
It was one of the area's biggest flare-ups since a ceasefire ended a 2006 war between Israel and Iran-backed Hezbollah.
Foreign Minister Annalena Baerbock said it was partly thanks to the UN peacekeepers that "this spark did not lead to a fire".
"Hezbollah is extending its influence and the Lebanese military remains unable to monitor the whole country's territory and the border with Israel," she said.
"It is not in our interests to simply look on."
Another "destabilising factor" is a growing wariness in Lebanon of hosting about 1.5 million refugees from Syria, the German parliament was told.
The German contribution to the 10,000-troop UN Interim Force in Lebanon (Unifil) is expected to cost about €32 million ($34.7 million) for the next year.
German training has allowed for Lebanon's navy to take over patrols in two coastal areas, Ms Baerbock said.
Defence Minister Boris Pistorius said the naval mission had a "high deterrence capability" against potential weapons smugglers.
"For Lebanon itself, but for Germany too, an enduring peace and stability in the Middle East is central," he said.
