From the Operation Crossroads Wiki:
“The ships carried sample amounts of fuel and ammunition, plus scientific instruments to measure air pressure, ship movement, and radiation. The live animals on some of the target ships were supplied by the support ship USS Burleson, which brought 200 pigs, 60 guinea pigs, 204 goats, 5,000 rats, 200 mice, and grains containing insects to be studied for genetic effects by the National Cancer Institute.”
Also, the ship was an actual US naval vessel, USS Prinz Eugen. It was sailable and a mixed German/US crew shepherded it the US for inspection of its equipment and capabilities after WWII. After movement from Boston to Philly, the German contingent of the crew was sent home. The plan was to sail the ship to the target area but the US crew couldn’t keep the power plant running. 11/12 boilers failed so the ship was towed to the Pacific through the Panama Canal. Basically, the ship was fueled for the trip and ended up being towed. The fuel was not drained.
From the Prinz Eugen wiki:
“She was towed to the Pacific via Philadelphia and the Panama Canal, departing on 3 March. The ship survived two atomic bomb blasts: Test Able, an air burst on 1 July 1946 and Test Baker, a submerged detonation on 25 July. Prinz Eugen was moored about 1,200 yards (1,100 m) from the epicenter of both blasts and was only lightly damaged by them; the Able blast only bent her foremast and broke the top of her main mast. She suffered no significant structural damage from the explosions but was thoroughly contaminated with radioactive fallout. The irradiated ship was towed to the Kwajalein Atoll in the central Pacific, where a small leak went unrepaired due to the radiation danger. On 29 August 1946, the US Navy decommissioned Prinz Eugen.”
The ship flipped over and sank before draining or other disposition (another test target, scuttling in deep water) could take place. Environmental concerns other than radiation contamination weren’t high priorities.