I saw this back in the summer of 1978 at the height of the Star Wars craze and wondered sadly why no action figure was in the shops, or I couldn't buy a plastic kit of the octopus attacking the Texas Rose; it was great fun to watch, and the poster was thrilling too (reproduced on the DVD cover). Now I'm an old git, I know why - even if my attention has shifted to the beautiful bone structure of Lea Brodie (who appeared in North Sea Hijack and Space 1999 but alas little else) somewhat. Yes, Doug McClure may be a clunky choice of lead, but he's more fun than anyone seems to remember and even adds a kind of glum gravity to the far-fetched proceedings when he has to. Peter Gilmore and Ashley Knight were then in nautical BBC productions - The Onedin Line and Treasure Island and a handful of great character actors spice things up.
The effects are surprisingly decent, from the aforementioned octopus attack, to a gargly swamp monster (which eats one of the Irish builders from Fawlty Towers), and a nifty diving bell. OK, so the sets are plain in comparison, and mainly consist of bits of Gozo (what a nice holiday the cast and crew must have had - they were all at it in the 70's presumably for tax reasons.) but they work.
The script and direction are what makes this picture work best. There's a tautness which is often lacking in monster films, and a knowing humour (which was already in evidence in 'At the Earth's Core' from the same team). A good bit of exciting family entertainment, which I reckon still works despite all the CGI advancements.
It's something of a cult film I believe - so it's a pity the DVD couln't have had more extras, such as the little 'making of' segment from 'Clapperboard' and a gallery of suitably garish posters. But never mind, the film is satisfying enough!