The visual effects were supervised by John Richardson and the miniature work is excellent. There is also a very welcome return of Roger Dicken to the series who produced the creature puppets.
The Texas Rose ship miniature was constructed by MasterModels in the UK. There was also a miniature version of the fullsize diving bell. The models were shot in the outdoor filming tank at Malta where the live action production was also based.
Below is John Brown Left and John Richardson prepping the Texas Rose miniature and the malevolent giant octopus creature puppet before shooting a scene.
The miniature creatures that appear in the film were designed built and operated by Roger Dicken and the Zargs are one of my all time favourite movie critters. They appear in the scene where they attempt to scale the walls of the city. Other creatures that make an appearance are some sort of plesiosaur and a fish finned sea monster called a Mogdaan.
In some shots a fullsize Zarg leg and foot was used with the actors as well as a full size giant octopus and tentacles. These larger scale creatures were built by John Richardson's team.
Along with the miniature work there are a number of matte painting shots in the film executed by Cliff Culley.