The video you're describing sounds like it could be from a popular film or documentary about shark attacks or ocean survival, but it's hard to pinpoint the exact source without more specific details. A scene somewhat similar to what you described occurs in the movie Open Water or in various documentaries about shark encounters. However, if it features a dramatic attack with blood and survival, it might also be from a more recent documentary or a viral video capturing a real-life event.
If you have any additional details about the context or characters in the video, it might help narrow it down further!