The Mess At Azov

[Feanor]

The Russian seizure of the Ukrainian warships is illegal under the UNCLOS, which both Russia and Ukraine have ratified. The said seizure is in effect an act of war because warships "have sovereign immunity even in another state’s territorial waters". That means “a state cannot seize a foreign warship in any circumstances, but it can expel it from its territorial waters”. Russia didn't expel the Ukrainian naval vessels instead seizing them and illegally detaining their crews in Moscow. Therefore regardless of the accusations and mudslinging between the two sides, Ukraine has the moral and legal high ground in this particular case.

Experts say Russia’s actions in the Kerch Strait were illegal

The relevant articles are:

Article 29
Definition of warships
For the purposes of this Convention, "warship" means a ship belonging to the armed forces of a State bearing the external marks distinguishing such ships of its nationality, under the command of an officer duly commissioned by the government of the State and whose name appears in the appropriate service list or its equivalent, and manned by a crew which is under regular armed forces discipline.​

Article 30
Non-compliance by warships with the laws and regulations of the coastal State
If any warship does not comply with the laws and regulations of the coastal State concerning passage through the territorial sea and disregards any request for compliance therewith which is made to it, the coastal State may require it to leave the territorial sea immediately.

Article 32
Immunities of warships and other government ships operated for non-commercial purposes
With such exceptions as are contained in subsection A and in articles 30 and 31, nothing in this Convention affects the immunities of warships and other government ships operated for non-commercial purposes.​

United Nations Convention on the Law of the Sea

This is very insightful, thank you for the research. This raises my question once again; why such a strong response from Russia?

Meanwhile, traffic has resumed as normal through the Kerch strait, after a brief delay allegedly due to either weather or a Russian blockade of Ukrainian ports (making the storm a complete coincidence).

Шторм закончился и российская "блокада Керченского пролива" была "снята"

CI Team did a breakdown of the timelines of Ukrainian and Russian statements. Unfortunately it's only in Russian, I'm hoping they will do an English translation like they normally do. The interesting thing is that the statements only appear to be contradictory, this timeline shows that the two stories can be reconciled, with each side choosing to emphasize what they want to, and ignoring what they would rather not have in the media.

Хронология инцидента в Керченском проливе по версии России и Украины — Conflict Intelligence Team

Bellingcat's take on the incident. According to them, Ukrainian ships did enter Russian waters off of the east coast, as well as the territorial waters off of Crimea.

Investigating The Kerch Strait Incident - bellingcat