On the 25th of November 2018 three Ukrainian military ships violated Russian territorial waters. These waters were Russian even before the reunification of Crimea with Russia. During the incident in the Kerch Strait Russian border guards reacted according to the rules of international law.
Recurring pro-Kremlin disinformation narrative about Ukraine violating Russian territorial integrity in the Kerch Strait in Azov.
The situation in the Sea of Azov was addressed by the 2003 bilateral agreement between Ukraine and Russia, which defined these territories as internal waters of the two states and gives both parties the power to inspect suspicious vessels; whereas both the 2003 agreement and UN Convention on the Law of the Sea provide for the freedom of navigation.
Crimea was illegally annexed by Russia in 2014, but the bilateral agreement on the Sea of Azov remains in power. Read more here.