I always wondered why Godzilla spends most of his time underwater when he's generally considered to be a mutated from a land based dinosaur; does he dwell in the sea for biological reasons to do with his radioactive nature?
Well, we have to remember there have been at least a couple of different origins for Godzilla over the decades. The original films treated Godzilla as a member of a "living fossil" dinosaur species that had survived undiscovered in the ocean depths for the 2 million years [sic] since the dinosaurs had died out. We know that this was the natural form of that species because there was more than one of them; the original Godzilla was killed at the end of the first film, and the Godzilla who debuted in Godzilla Raids Again
and continued through the rest of the Showa Era was explicitly described by Dr. Yamane as dai-ni no Gojira
, "a second Godzilla." Plus we later met a third, juvenile member of the species, Minya. So the original idea wasn't so much that the Marshall Islands atomic tests had mutated the Godzillae as that it had displaced them from their natural aquatic habitat, causing them to storm onto land in search of new food sources, and had also contaminated them with radioactivity, which they were somehow able to direct through their breath and focus into a weapon.
(Compare the 1955 Harryhausen film It Came from Beneath the Sea
. The giant octopus in that film was also treated as a naturally occurring giant that had been radioactively contaminated by the exact same Marshall Islands tests, enabling the fish it preyed on to sense its radioactivity and flee, and thus requiring it to leave its feeding grounds in search of new, less radiosensitive food sources like human beings. See also the 1954 Them!
, the first movie about giant mutant animals created
by radiation; in that film, radiation didn't make individual ants grow larger, but had gradually mutated an ant species into progressively larger forms over many generations since the earliest nuclear tests in 1945.)
It wasn't until the 1984 reboot that Godzilla was reinterpreted as a creature that fed on and was powered by radioactivity (shifting the allegorical focus from atomic weapons testing to the nuclear power plants which Godzilla now targeted and fed upon), and it wasn't until the 1991 installment of the reboot (Heisei) continuity, Godzilla vs. King Ghidorah
, that we got the revised origin wherein Godzilla was originally a surviving, land-dwelling carnosaur that was only about twice the size of a T. rex until he was exposed to radioactivity and transformed into a giant. The Heisei series assumed that exposure to radiation would consistently transform surviving dinosaurs into giant forms; the same thing happened with Rodan and with Baby Godzilla/Godzilla Junior. But the Heisei films were also quite inconsistent on the question of whether the post-1954 Godzilla was the original somehow returned from the dead or a second member of the same species (in which case having them both undergo the same mutation would be rather implausible). The first couple of Heisei films were ambiguous, the third assumed it was the original, but the concluding film of the series explicitly indicated that it was a second, distinct Godzilla. (My best rationalization is that they originally believed it to be the original but later figured out it was a separate one.)