Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The Sea of Galilee is not a sign of the appearance of the Mahdi. Rather, the Sunnah mentions that its drying up will occur during later events among the signs of the Hour, at the time of the emergence of Gog and Magog during the era of Jesus, peace be upon him.
It is authentically established in Sahih Muslim, in the hadith of An-Nawwas ibn Sam'an, may Allah be pleased with him, that the Prophet ﷺ mentioned the emergence of Gog and Magog and said:
“Their first ones will pass by the Sea of Galilee and drink all the water in it, and their last ones will pass by it and say: ‘There was once water here.’”
The emergence of Gog and Magog will occur after the descent of Jesus, peace be upon him, and his killing of the Antichrist (the Dajjal). Allah will then reveal to Jesus, peace be upon him, to take the believers to the Mount. Gog and Magog will then emerge and drink the water of the Sea of Galilee until it becomes dry.
Then Allah, Exalted is He, will destroy them through the supplication of Jesus, peace be upon him, and will send against them the naghaf in their necks. After that, the earth will be cleansed of their آثار.
Therefore, it is not correct to connect the drying up of the Sea of Galilee with the appearance of the Mahdi, nor to regard it as an independent sign that his appearance is near. Rather, the text that has been transmitted connects its drying up with the emergence of Gog and Magog during the time of Jesus, peace be upon him, which is one of the later events of the end times.
And Allah, the Exalted, knows best.