The first film establishes the mythos around Jason, but he is only a young boy who drowns in the lake. The killer in the first movie is actually Mrs. Voorhees, Jason's mother, who is seeking revenge for her son's death.
It's in Friday the 13th Part 2 (1981) that Jason first appears as the iconic hockey mask-wearing killer, and he is physically deformed due to his time spent at the bottom of the lake.