Now if you are more ambitious you can make additional modifications to increase the range and depth. The PDF file linked in this RC Universe forum thread entitled "Motorworks Submarine Modification Manual" details fairly easy steps to improving the performance of this popular toy submarine. However, it does require opening up the sub (beyond just opening the battery compartment).
While many of the internal parts of a toy sub won't be up to snuff for serious RC subbing, you could junk most of the internal parts and just use the hull as a container when building your own RC submarine from scratch.
With hobby-grade Ready-to-Run or built from kit submarines you can do a lot more. Add a wireless camera, torpedo launching systems, more working parts (periscopes, hatches, lights), or change the motor. The sky's the limit. Be careful though -- you also have to consider weight issues. You do not want to have a rock for a sub after adding a lot of extra components -- that would make for a troublesome, expensive and not to mention boring experience just sitting on the bottom of the lake.