Assuming that there is an Earth-like planet that has 8 times the volume, 2.83 times the radius, and has the same surface gravity as Earth, and has oceans; what are potential problems concerning the density of this planet if it isn't hollow that risk it being less Earth like either composition-wise or just in how hard or soft the surface is?
To shed mass you would have to lose your iron/molten core and the protective magnetic field it creates to shield your atmosphere from solar winds. And without a molten core you wouldn't have enough volcanic activity to replenish your atmosphere.
As a solution you could have a core of metallic hydrogen (~%7 the density of iron). It would allow you to maintain your magnetosphere while achieving your desired volume and gravity without significantly changing the surface characteristics of the mantle.