Russia effectively began blockading Ukrainian ports in the Sea of Azov earlier today when sealing the Kerch Strait by placing a merchant vessel across the sea lane passing under the Kerch Strait bridge and forcing a small convoy of Ukrainian vessels to head back. The incident included vessels of the two sides making contact with limited damage. However, pictures have surfaced of another Russian Border Guard vessel with damage apparently from a collision, and it is unclear if more vessels than originally reported were involved, if the incident is unrelated, or if the pictures are old.
However, a while ago reports started coming in that a firefight have taken place. Apparently the first casualty was Ukrainian patrol craft (gun) Berdyansk (pennant U175) which reportedly lost propulsion. After that Russian forces tried boarding the vessel, with Berdyansk returning fire.
Exactly what has taken place since is even more unclear, but it should be remembered that the closing of the strait lead to a significant number of civilian vessels being stuck in the area waiting for things to clear up that they could continue their journeys. There is a very real risk for these, including both Russian and Ukrainian ones, being caught in crossfire. Significant air activity was also observed throughout the day, including Su-25 attack aircraft and armed Ka-52 attack helicopters. One report stated that following the exchange of fire both Berdyansk, sister Nikopol, and a naval tug has been captured by Russian forces, but currently the word is that six Ukranians are wounded, two vessels are under tow by friendlies, and one vessel is held by Russian forces.
Unconfirmed reports have stated that the Ukrainian Navy has left its base in Odessa, but it is very unclear if this indeed has happened, what vessels are at sea, and if there is some battleplan. The Russian Black Sea Fleet together with air and ground units will have no problem stopping the Ukrainian Navy if they try to force passage through the strait. The sole major surface combatant of the Ukrainian Navy is the Krivak III-class frigate Hetman Sahaydachniy (U130). It should be noted that the Krivak III was the coast guard-version of the class, and while very heavily armed for a coast guard vessel, it still lacks any kind of anti-ship missiles. The Ukrainian Navy has a number of older fast attack craft as well, but their operational status is unclear. If this motley flotilla is supposed to survive, let alone do any damage to their Russian counterparts, it will need some serious air support.
Both Ukraine and Russia have large numbers of aircraft in the region, including Su-24M which while old still can do serious damage to surface units, especially as the target vessels in many cases are old as well with limited air defences (though it should be noted that the Russian Black Sea Fleet include a number of modern corvettes and frigates which likely will eat Su-24s for breakfast). For Ukraine the question is where any potential battle would take place, as the Kerch Strait is ‘behind’ occupied Crimea. If Ukraine is to secure even limited air superiority, the battle will likely have to take place somewhere else, which might require the Russian Navy cooperating. Another question is if things could now deescalate as it seems the active battle at the strait has died down following the Russian capture of one of the vessels involved? There is no longer an urgency on the part of the Ukrainian to rush headlong into the waiting Russian forces.
On the other hand, the wheels might already be in motion, and a serious question is at what level command currently rests on both sides? If the politicians have transferred operational decisions to the military things could keep on escalating. In the same way, decisions by local commanders in the field can have outsized impacts upon the continued developments on the Russo-Ukrainian War as a whole. This would not be the first time that politicians have had to come to grips with the fact that measured escalation is difficult.
In the end, a conflict over Russia blockading the strait has been one of the scenarios discussed numerous times since 2014, and as predicted it shows signs of escalating easily. A crucial factor regarding the timing is that Europe is focused upon Brexit, diminishing the potential of EU to work as a stabilising factor. At the same time, it should be remembered that early reports seldom are to be trusted, and by tomorrow morning we should all be wiser.
Corporal Frisk 
1 gunboat and 1 tug reported to have been boarded while on the gunboat Nikopol the crew appears to have barricaded themselves inside while the vessel is being towed. Unconfirmed report that the Ukrainian navy has left the base in Odessa.
It seems there was a collision between the Russian border guard ships. Radio communication between the Russian leadership and border guard ships: https://www.youtube.com/watch?time_continue=8&v=xpdlGAweVu4 (at 2:24)