problem about past contiuous

絕對可以! Absolutely correct!!!
The boys took photos while they were waiting for the ferry.
= The boys were waiting for the ferry when they took photos.
It depends on what the main focus is!
Usually we put the main point/ focus at the beginning as the main clause
要看句子的重點. 英文通常的做法是重點寫成主句, 放在句前。較不重要的寫成子句 sub-bordinate clause
The boys took photos ---> main clause / focus
while they were waiting for the ferry -----> sub-bordinate clause
The boys were waitinf for the ferry----> main clause/ focus
when they took photos ------> sub-bordinate clasue

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WHEN唔可以用係後面. 要放係前的.
the boys were waiting for the ferry when they took photos.

Should be:
When the boys were waiting for the ferry, they take the time to take photos.

因為如果個WHEN (當) 用在後面係冇左個意思. 如果呢句係中文. 你都唔會放個當在影相果度.