Trying to keep the water from getting in between the spool and sideplates would be a major issue
. In order to get a good seal, the reel would have no freespool.
Any other place could use o-rings or gaskets. Okay for rock-codding with 3 pound weights but in any other application, I see headaches.
Or you could simply keep it in a Ziplock bag
I think that a reel needs to be able to let the water that gets in drain out. Having a "waterproof" reel might work against you, especially if some water gets in and it can't come out. Just my opinion