How to get a large ocean map (is it even possible)? [closed]

Chances are that you have spawned in a different place. Your seed does not determine where you spawn in a world, it is random, and given the scarcity of sand in this map you've probably spawned a way away. Ask the person on the forum thread to post their coordinates (visible by pressing F3) and walk to the same spot in your game.

As for the size of oceans, I don't think there's an upper bound, although ones the size of your picture will be very rare. If all you want is a massive ocean, use MCEdit to make one.

UPDATE

I followed the instructions on the thread, looks like I got the world you're talking about:

Separate to my other answer, the biggest ocean seed I can find is this one:

Timestamp: 2011-03-02 06:55:36 U

As seen in this MC Forum thread.

As of version 1.8, there appears to be either a bug or an undocumented feature that causes some seeds to generate "water worlds" with only small, scattered islands for days of flying (in creative mode) in any direction.

One such seed that I've personally tested is 1335977250. Nothing but islands for days around the spawn, and I only ever saw two trees. It's perfect as a blank canvas for a water world project.

As of version 1.8, there is a new ocean biome, which means that the world generator spawns huge oceans. In my world (seed Tom) I spawned next to an ocean that, once I sailed into it with a boat, filled my entire world (all I could see from horizon to horizon was water).