Lake or Sea

By flying around, using F2.
You could use Seed "Lake" to spawn directly at a very big lake.
The coordinates for this lake are 100 100 (height ca. 43).
I've found a lake near 100 100 on a Multiplayer Map, too, maybe you could try it.

or use the seed "Australia" to get a very darn impressive lake

Quote from zfoxfire

or use the seed "Australia" to get a very darn impressive lake

lol

when you create a new world there is a textfield for the seed, by default it has a number in it but you can change that to w/e you want

Lol .... I spawn in large sea ;D