What often provides early information on casualties in a ship investigation?

The early information on casualties during a ship investigation is often obtained from pressure gauges on steam or water lines. These gauges can provide immediate insights into the integrity of systems that are vital for the vessel's operation. If there is a breach or failure in these systems, the pressure readings can drop significantly or fluctuate, indicating potential damage or flooding.

Moreover, pressure gauges are effective tools for monitoring changes in the environment that can lead to serious issues, such as a loss of coolant or steam pressure, which could suggest damage to critical machinery or piping systems. The data from these gauges aids investigators in assessing the situation more rapidly and accurately, which is crucial for coordinating rescue efforts and implementing damage control measures.

While other options like temperature readings, visual inspections, and sound alarms could provide valuable input during an investigation, the direct and immediate feedback obtained from pressure gauges is particularly significant in assessing the extent of damage and the potential for casualties.