The first indication of disaster was around 6 pm when the winds dropped and the sky grew darker. This can be identified as the first indication of disaster based on the given options. Let us discuss this in detail:

1. Around 6 pm when winds dropped and sky grew darker:
- This is the most appropriate answer as it suggests a change in weather conditions, indicating a possible disaster.
- When winds drop and the sky grows darker, it usually signifies the approach of a storm or severe weather conditions.
- These changes in the atmosphere can be early signs of an impending disaster, such as a hurricane, tornado, or heavy storm.
- The drop in winds and darkening of the sky indicate that the weather is worsening and could potentially lead to a disaster.

2. Next morning when the ship started creaking:
- While the ship starting to creak can be a sign of danger, it is not the first indication of disaster as mentioned in the options.
- The creaking of the ship might be due to various factors such as rough seas, strong winds, or structural issues.
- However, it does not directly indicate a disaster unless there are other accompanying signs.

3. When it started raining:
- The start of rain alone is not a definitive indication of disaster.
- Rainfall is a common occurrence and does not necessarily imply a disaster.
- It is possible for rain to be heavy or accompanied by thunderstorms, but without further information, it cannot be considered the first indication of disaster.

4. When winds were strong:
- Strong winds can be a sign of potential danger, but it is not the first indication of disaster as stated in the options.
- Strong winds can occur in various weather conditions and do not necessarily indicate an immediate disaster.
- It is important to consider other factors, such as the drop in winds and darkening of the sky, to determine the likelihood of a disaster.

In conclusion, the first indication of disaster in this scenario is around 6 pm when the winds dropped and the sky grew darker. This change in weather conditions suggests the possibility of an impending disaster, such as a storm or severe weather event. The other options mentioned are not the first indications of disaster or lack sufficient information to determine their significance.