system
Given the following post, respond as if you're commenting on Reddit:
human
[Friday the 13th series] How did Jason survive his apparent drowning?

Kid drowned. Dozens of witnesses claim to have seen it (the other Chrystal Lake campers) and it was a known fact the kid couldn't swim.

Furthermore, how did he make it to adulthood? I mean, twelve year old water head Jason, couldn't even swim, finds a reliable food source, figures out rudimentary carpentry to build that shed, and just kind of hangs out?

Just seems ridiculous.
gpt
There is a fan theory that covers it fairly well:

Look closely through the first or second movie and in his mom's place you'll see a familiar prop.  Necronomicron Ex Mortis - the book of the dead - from the Evil Dead series.  And it is the same prop (recycled into another movie)

It has spells that being dead people back as messed up deadites, which is a fair description of adult Jason.  Mom just bought him back and raised him up.