The major difference between the Army’s AH-1G Cobra, and the Marines’ AH-1J Sea Cobra was the substitution of the G’s single T53 engine for the T400, basically two Pratt & Whitney Canada PT6 engines running as a single power pack. The chin turret with twin 7.62 Miniguns was replaced by the XM197 three barreled 20mm cannon.
The T model for the Marines added TOW missile capability, but was otherwise virtually identical. The J/T SeaCobras would eventually be replaced by the AH-1W, with far more powerful engines and the ability to fire Hellfire missiles, and they themselves have recently been replaced by the AH-1Z, itself so different from the original Cobra series as to be pretty much an entirely different helicopter, sharing only the same general outline.
It should be noted that while the Cobra and the Sea Cobra are quite similar, the Army and the Marines actually developed quite different doctrines of employment for attack helicopters. That’s because the Army and the Marines have different roles and missions. Neither doctrine should be seen as superior or inferior to the other, but as tailored to support its services needs.