The captain knew that another attempt would be made later, for the sunken ship he was trying to find had been carrying a precious cargo of gold bullion.

1. 为什么要用过去完成式？

【答】你说的had been carrying 应该是“过去完成进行时”。该时态的基本用法是：

“过去完成进行时”，表示动作从过去某一个时间之前已经开始（例如上周五），一直持续到过去这个时间（例如昨天上午）仍在进行，那么，动词时态应使用过去完成进行时。过去完成进行时的用法与现在完成进行时的用法，道理基本相同，只是时间参照点不同。过去完成进行时以“过去的过去”为时间参照点，而现在完成进行时以“过去”为时间参照点。过去完成进行时指从“过去的过去”某一个时间点开始的动作或状态，一直延续到过去某个时间，而且当时仍在进行中。例如：

    I had been waiting in the room for half an hour before she turned up.

    在她出现之前，我在房间里等了半个小时。

    Sorry, I didn't know you had been waiting for me.

    对不起，我当时不知道你一直在等我。

    Amy was tired out because she had been studying since six o'clock.

    艾米筋疲力尽，因为从6点钟开始她就一直在学习。

    My daughter had been trying to find a job,but didn't find one.

    我女儿一直在设法找工作，但没找到。

涉及到本句语境：船长过去在寻找那条沉船，而“该沉船在此之前一直运载着一批珍贵的金条”。不仅要表达出“过去的过去”，还要在进行中。

2. 能改成was carrying 吗？

下面是我自己造的一个句子：The man the police was trying to find was wearing a black coat.

这句话应该不会用had been wearing 吧。

【答】都对。只是意思稍有差别。即便用一般过去时也对（The man the police was trying to find wore a black coat.），主要原因是：wear 这个词表示状态，一般式和进行时区别不大。