How long can my apple watch be in the water

Your watch will definitely be able to handle that. Make sure to put it in water lock. You can do that by swiping up front the clock face, and tapping on the water droplet. This will prevent the screen from registering touches (water seems like a touch to electronics). I would also test the watch gradually, maybe wear it for an hour and then check on it.

Hope this helps!

If it's the series 5, it should be able to withstand being in the water for quite some time. It is meant to go swimming with after all. Did it fall into a lake or something?