Is it permissible to take a bag of fish from the ship's captain, sell it, and benefit from its price, with the apprehension that the captain might not be authorized to do so?

The fundamental principle is that a person's actions concerning their own property are valid and permissible, and dealings with them are not prohibited unless there is evidence proving they are disposing of someone else's property without permission. Unless the matter reaches a point of strong suspicion that they are disposing of someone else's property without permission, it is impermissible to deal with them. However, mere doubt and conjecture are not taken into consideration, for possession is proof of ownership.