the main thing you want to get is called Amidst. download it. it runs outside of minecraft, so no modding installation.
you type in your seed number, and it will show you the entire map generation.
after that you just drag the map around like using google maps or w/e.
if you dont want to use any mod, or mapping, i would pick a direction and start travelling. eventually you will hit a real ocean.
the "ocean" in my seed where you can find mushroom biomes is something crazy like: 30,000 blocks across each direction.
...30,000 blocks across... is a real ocean. lol