There are several ways to watch Voyage to the Bottom of the Sea legally online or via download either free with advertising (Hulu) or for a small fee per episode (Amazon).
Many Irwin Allen shows are available on Hulu.com, but unfortunately at the present time these are only available to US viewers. Hulu also reportedly have plans to start charging for online viewing in the future. The first two seasons of Voyage to the Bottom of the Sea are available to view on HULU.
The search box at the top of the Hulu window allows you to search for other episodes and Irwin Allen shows. When the video is played, an icon appears at the top right corner which takes the video to full screen when selected (Press ESC key to return to normal browser view).
Note, if you live outside the US, the Hulu video will not play.
If you live in the US, episodes can be purchased and downloaded from Amazon.com. Click on the image link below for more details.
Click on the image above for Amazon UK Instant Video Purchase Options (First Season Only)
Voyage to the Bottom of the Sea Season One is available on ITUNES. The good news is that Lost in Space, The Time Tunnel and Land of the Giants are also available to UK viewers for £1.89 per episode, so it is worth having the latest ITUNES software in order to purchase episodes of interest.