As,I'm sure you know, oceans are very big. A drop is a very small quantity of liquid.
Would you like another drink?
Just a drop then.
Tear drops will fall and so will rain drops.
A drop in the ocean is an insignificant amount; not enough. They raised some money for the famine in Africa but it was just a drop in the ocean.
What if you literally took a drop in the ocean; what would it look like? Something like this. Thanks to my diving student Krysza for this idea.
BTW By the way, this guy's only oxygen supply is what he has in his lungs, Truly amazing - I pass out just watching.