Hi Matt,
Finding a ready to go long-range RFID setup may be easier said than done. The setup would be akin to the anti-theft gates at supermarkets which is quite high powered and cumbersome. Here is an article explaining those anti theft gates if you really want to go down that path:
In any case, you could get the RFID tags and a basic reader and if you wanted to update to a longer distance reader in the future you could after you had gotten the system working. The RFID tags we sell aren’t waterproof but it shouldn’t be too hard to waterproof the ICs. Here are a few examples:











