I put a water sealant on mine, which were old boards that had been shelves in the garage. They have been used for 4 years now. Where I live, the major problem would be termites more than rot.

I put pure raw lineseed oil on mine but I have only used them for 1 year so I do not know if it is worth the time. I used pine 2x10's and they are so cheap it may be easier to replace them when needed rather than use the lineseed oil. I will have to wait to see if they last longer than the one I did not put the lineseed oil on.

May not look pretty but you could wrap them with plastic. Just don't use the plastic for the bottom of the bed. Paint the boards a nice color and wrap with clear plastic.
Paws

I used a 2 X 8 X 12 12 years ago, nothing on it to preserve it.I may get another year out of it, open on one side apparently means a lot

I use a product called eco-wood. it's non toxic and supposed to protect wood for a long time