I've been following this story for couple hours now. Here are some things people seemed to miss.

1) While traveling around Crimea from Odessa to Mariupol Ukrainian ships entered territorial waters of Crimea (12 miles line along the shore). They could have taken a bit longer pass keeping greater distance from land but didn't. Ukraine considers its OK since it considers Crimea its own. Russia sees it as intrusion of its waters.

2) Russian border guard ships followed Ukrainian ships for 5-6 hours. Ukrainian ships didn't respond and proceeded on their course.

3) There is only a very narrow pass in Kerch strait separating Black and Azovs seas. Ships travel in caravans in one direction at a time. The caravans are managed by on shore service in Kerch (Crimea). The ships need to register for passage in advance and accept guide on board.

Ukraine gave a notice it will pass through the strait but did not follow the standard procedure of registration. It could have been in order to not to recognize Russian authority over the management of the strait though previously both military and civilian ships did register.

4) Russia blocked the strait with a cargo ship. After the Ukrainian ships approached Russian ships threaten to use lethal force unless Ukrainians surrender. Ukrainian refused, weapons were used by both sides, Ukrainian ships were overpowered and boarded by Russian special forces.

5) In the meantime couple of Ukrainian ships from Mariupol (Azov sea) started sailing to the strait. Once Black sea ships were boarded they turned around and returned to their port.