Interesting subject here.
The only difference between these two is that diving is more direct than plunging. You can plunge by accident, perhaps if you fall into the sea or a swimming pool. When you dive you intentionally do so, and you are in control of the situation.
- John stepped up onto the diving board and dived into the pool.
- John went too close to the edge of the boat and before he could shout for help, he plunged into the ocean.
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