After 10 minutes of boating, the way of block destroying is glitchy.. Minecraft "thinks" that there still exists a block, which I destroyed and I do not see it. I know, what Minecraft "thinks", because I can not get to the place, where "my" block was and when I reload my map, "my" block appears in his place.
I made a video of that:
https://www.youtube.com/watch?v=ovBsbkLyAuo
(sorry for my poor english)