[@Haeo] I don't know how to properly and scientifically calculate Life expectancy, but people tend to live to be 70-80 years old is they are healthy and get enough food/water. The medical tech is basically zero, so things like cancer, diabetes, heart conditions, and things that require surgery or medication to fix end up just killing people now. Malnutrition, starvation, dehydration, and drowning are the number one killers in Waterworld. 

As for how people understand the passage of time, its all based on the fazes of the moon. Every new moon is considered to be the beginning of a new month. Every 12 new moons, is a new year. So "September" is the ninth new moon of the year. A new moon is when there is no moon in the sky. The sun and shadows are used to figure out what time of the day it is, for instance the sun at its peak, and no shadow means it is noon. 

I really hope you get to play!