This has been happening to me since 14w31a but it is happening so much that I can't even play on Minecraft. This has been happening on an island that I imported via software called mcedit but I haven't checked if this happens in other places in my world. All I know is that it will happen randomly. It could happen when I start walking (Instantly upon loading world) or at any point after that. Also, the game might just freeze in place instead of crashing like it should. It even prevents me from force quoting the crashed Java session until I restart my computer (Other wise, I just have to let it sit in the dock while using other applications). The longest I was able to play before the crash happens was about 12 minutes. I'm not sure if this has happened to others and I do apologize if this is already an issue. Could it have something to do with the ocean perhaps?