you have to calculate a volume. this requires calculating the surface area of the earth, taking 70% of that, and multiplying by the average depth of the ocean. Once you have that volume, it is a simple thing to multiply the volume by the density of sea water to achieve the mass (order of magnitude or ballpark figure based on your estimated area and depth values).
In fact, given the other uncertainties in your values, you can simply use one cubic meter of water equals one metric tonne (1000 kg) and ignore the couple percent difference between fresh and salt water density because your value is only a gross estimate no matter how you cut it.
I assume you know how to calculate a surface area of a sphere, and that you can look up a value for the radius of the earth.