A drop in the ocean (3,4)

This could be a double definition.
'a drop' is the first definition.
The answer and definition can be both related to natural events as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'ocean' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for low tide that I've seen before include "when recession at its most acute?" , "Condition of sea at its least high level" , "Farthest ebb of the waves" .)
(Other definitions for splashdown that I've seen before include "A drop in the ocean" , "return of spacecraft" , "Landing of spacecraft on water" , "Spacecraft's landing at sea" , "End of mission" .)
'a drop in the ocean?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ebb that I've seen before include "Initially Mrs Browning's" , "Go out like tide (3)" , "(Sea) recede" , "Withdraw heartless" , "Drain away; decline" .)