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.
If you live in the US, first season episodes can be purchased and downloaded from Amazon.com. Click here for more details (#ad).
Voyage to the Bottom of the Sea TV series DVD set DVD and first season online streaming are available at Amazon.co.uk (#ad).
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.