Answer:
The sentence presents the actions in correct sequential order, it is proper to express in Simple Past Tense as follows :—
I accidentally dropped my phone in the (+) water and I (+) tried to save it but the damage had already been done.
If you want to use ' had accidentally dropped ’, you may consider as follows :—
I tried to retrieve (alt.) my phone which I had accidentally dropped in the water but the damage had already been done.
You can also put in some passive voice as follows :—
I tried to retrieve my phone which had accidentally been dropped in the water, but what good is it ? The damage had already been done