I fixed a redwood hottub several years ago with clear silicone seal. The leak may be in the groove where the staves contact the bottom. Water gets in there (from anywhere) and can travel in the groove. I drilled from outside into the groove and used a tube of clear silicone seal injected with a caulking gun. You may need to do this every 12 inches or so. The tub does not need to be empty. Good luck.