The moon is just the right distance away from the earth to affect the tides.
If the moon were farther, the stagnant water wouldn't be able to support plant life;
if closer, the tides would cover too much of the land.
Underwater earthquakes occur in just the right amount to disperse the nutrients lying on the seabed without destroying too much of the ocean life.
The salinity of the ocean is also a key characteristic; if the ocean wasn't salty, ice would form more easily, reflecting heat back out into space and keeping the planet in a permanent ice age.