I am about to purchase either a 74sv or 94sv. I looked at both in the store today and the bluechart g3 coastal maps that come with the units look pretty basic. Is there other mapping software that is better, maybe that shows the wrecks, reefs, or other structure? The reason I am asking is that I can save quite a bit by purchasing a 73sv and even with purchasing an updated coastal map still have no more into it that getting the 74 sv. Thanks.