Literal Translation
It’s not like you have to drink the ocean
Actual Meaning
It’s not that difficult
Etymology
17th century, found in Jean de La Fontaine’s fable Les Deux Chiens et l’Âne mort (The Two Dogs and the Dead Donkey). He uses it in the affirmative form, saying «Si j’arrondissais mes États! Si je pouvais remplir mes coffres de ducats! Si j’apprenais l’hébreu, les sciences, l’histoire! Tout cela, c’est la mer à boire», which translates to “If I could round off my states! If I could fill my coffers with ducats! If I could learn Hebrew, science, history! All this is an impossible task.”
The metaphor refers to an impossible undertaking, ‘at the cost of often insurmountable difficulties (such as, for example, attending a two-hour lecture on beetroot cultivation in the plains of Tajikistan without yawning),’ as per Georges Planelles’s witty analysis.
As is often the case in the French language, the way this expression is used has evolved. Nowadays, it is only used in the negative form. This highlights the supposed laziness of the target, who makes a big deal out of an insignificant task. So when I say “I can’t do X it’s just too hard” you could say «c’est pas la mer à boire!»… Whether or not that is advisable is a different question entirely!