Whenever you have a device such as a smartwatch that is so predicated on the use of another device, it raises the question of whether or not that device can work on its own. When talking about best hybrid smartwatch models, no, they will not need the use of your smartphone to function. That is, they will still be able to keep track and time and function as an ordinary watch.
But, if you want to utilize the smart features then yes, it needs to be synced to your smartphone. If you are merely going for a stroll and do not require the services of Bluetooth, though, it can be used without it. This is a good practice anyway as disabling Bluetooth can end up saving on the battery in the long run.