Here is a possible concise summary:
The text is a transcript of a video about SCP-1861, a paranormal submarine that appears in bodies of water and causes a red mist and rain of blood. The video tells the story of Sean, who almost boarded the submarine, and Sal, who did and became one of the SCP-1861-B entities that wear diving suits and try to recruit others. The video also reveals some of the Foundation's research on SCP-1861, such as its spatial anomaly, its alternate dimension of living water, and its mysterious watcher of eyes and biter of teeth. The video ends with a warning to avoid SCP-1861 at all costs and some suggestions for other SCP videos.
Here are some key facts extracted from the text. I have numbered them and kept them short sentences.
1. SCP-1861 is a World War One era British submarine that appears near bodies of water and causes a red mist and rain of blood.
2. SCP-1861-B are men in diving suits who try to persuade people to board the submarine for their safety.
3. The first recorded incident of SCP-1861 occurred in 1916 in a British town called Innsmouth on Sea, where the entire population vanished.
4. The Foundation wiped all records of the town and administered amnestics to anyone who remembered it.
5. The interior of the submarine is a spatial anomaly that can fit hundreds of people and leads to an alternate dimension of living water.
6. People who board the submarine are forced to wear diving suits that transform them into new instances of SCP-1861-B.
7. The alternate dimension is populated by dead humans and animals with missing eyes and mouths, which are attributed to a mysterious being called the Watcher of Eyes and Biter of Teeth.
8. The diving suits are impossible to remove by anyone other than the wearer, and anyone who does so dissolves into the water.
9. The Foundation has classified SCP-1861 as Keter and tries to intercept it whenever it appears and prevent civilian kidnappings.