Godzilla is 6x longer than the Megalodon and breathes atomic fire — this is a mismatch.
The Megalodon was the largest shark ever at 60 feet. Godzilla stands 400 feet tall. Even underwater — the Megalodon's home turf — Godzilla's sheer size, atomic breath, and nuclear-powered durability make this completely one-sided.
Godzilla has fought and defeated other massive aquatic creatures in his universe. The Megalodon's impressive bite force (40,000+ pounds) is formidable but meaningless against Godzilla's armor that withstands nuclear weapons.
Atomic breath underwater would create a superheated shockwave that could kill the Megalodon without direct contact.
Godzilla is 6x larger, breathes atomic fire, and has nuclear-powered regeneration. His armor withstands nuclear bombs. The Megalodon's best weapon — its bite — can't penetrate Godzilla's hide.
The Megalodon was the ocean's ultimate predator with the strongest bite in history. In deep water, it has superior maneuverability for its size. But the size gap is simply too extreme.
Godzilla wins with minimal effort. The Megalodon is the most fearsome shark ever, but Godzilla is a nuclear-powered kaiju hundreds of feet taller. Different leagues entirely.
Godzilla also fights
Megalodon also fights