Why does water have a moderating effect on climates?

Because water has a tremendous capacity for holding heat ... much more than land, and much much much more than air.

Since water can hold so much heat . . . and it can only absorb heat or radiate heat
from its surface . . . it takes water a long time to get warmed up in the morning (or
even over several warm days), and a long time to get cooled down at night (or even
over several cold days). 

So, say it's the beginning of Summer and getting hot.  If there's a large body of
water near you, the water is still cool for several days, and air blowing over it
gets cooled before it blows over the city.

Now, say it's the beginning of Winter and getting cold.  If there's a large body of
water near you, the water is still warm for several days, and the air blowing over
it gets warmed before it blows over the city.

That's why places near the ocean coast, or the shore of large lakes, have more
"moderate" climates . . . the Summers there are less hot, the Winters there are
less cold, and the temperature doesn't change fast, 

IF THERE'S A LACK OF WATER...IT IS LIKE A DESERT AND VERY DRY
LET'S TAKE HAWAII AS A EXAMPLEBY THE OCEAN SO IT IS HUMID