It all depends on the density of the rock. If it is very porous, like Marshall Island, Fiji or Kaelini, then the hammer/chisel will work. BTW, if you use a wood chisel you stand a good chance of damaging the sharp edge. What you should use is called a mason's 'cold chisel', it's designed for exactly what you're intending to use it for.
If the rock is more dense, like Caribbean, then you might have to drill a series of holes then use the hammer/chisel method.
Also, pre-drilling some holes in strategic places can help you sculpt the rock to help you aquascape more precisely.
swimmer