my teenager sim died from going in the ocean water

Was her hunger or energy low?

Were any of her needs in the red at the time? If one or more needs are in the red I believe it can trigger a drowning (particularly Hunger/Energy). Did an event happen? Like did she interact with a mermaid? Was it overly hot? I know you said it wasn't summer but you can still sometimes get heat waves in Spring I would assume this might be more common on Sulani.

There was a warning about seeing a shadow in the water in yesterdays stream - maybe she encountered a shark - did she see a shadow in the water. Maybe a run in with jelly fish or even an angry mermaid.

all i can think of is she was angry ill have to look at it again

To the point of being enraged? Maybe she was supposed to die of anger, but there was no animation for it in the water, so the game got confused and just immediately killed her?

Teens aren't supposed to be able to die from emotional deaths because they get mood swings that make them Enraged or Mortified if you have Parenthood. Of course it could have just glitched out somehow if she went into the ocean while Enraged.

This happened to one of my sims a little while ago. It turned out it was a shark in the area. I quit without saving because I can't bear to lose my Brothers Green.