C’est pas la mer à boire means that something is not so difficult, that it is not a big deal. Literally? “It is not the sea to drink”. So, next time someone complains about a task you can say that whatever they have to do, Ce n’est pas la mer à boire, it isn’t as difficult as drinking the entire sea!
The expression dates back to the 17th century and has been used, among other writers, by Jean de la Fontaine in “Les deux chiens et l’âne mort”.