Why is Earth called a terrestrial planet even though it is 70% covered by water?
There really isn't a lot of water on the Earth. The image below, from the USGS science school, graphically illustrates the volume of the water on (and in) the crust of the Earth, relative to the volume of everything else.
The bigger blue ball is all the salt water, from all the oceans & seas, the small one is all the fresh water, from the rivers, lakes and aquifers. You may need to zoom in to see the fresh water ball...
There is more water dissolved in the mantle, but we can't get at it (with current technology), apart from what's released in volcanic eruptions.
From John Dvorak's comment, the word "terrestrial" derives from the Latin "terrestris", meaning "of or pertaining to the Earth", which itself is related to the Latin word "terra" meaning "Earth". So Earth is by definition a terrestrial planet.
As PM 2Ring notes, the quantity of water in the Earth is relatively small, corresponding to roughly 0.02% of the mass of the planet. There may be a similar amount in the mantle that's not accounted for in the estimate linked there, but it is still a very small proportion of the mass. The majority of the Earth is in the form of rock, with the second component being the iron/nickel core accounting for 33% of the mass.
The term "ocean planet" is often used to refer to planets that are scaled-up versions of Ganymede or Pluto that have migrated into the habitable zone, see for example Léger et al. (2004) 'A new family of planets? "Ocean-Planets"'. These planets contain far greater quantities of water: the case considered in that paper has 50% water by mass, similar to Ganymede. In particular the base of the ocean on such planets is not the water/rock boundary as it is on Earth: instead the ocean is separated from the rocky core by a layer of high-pressure ice.