The aux belt has to come off to replace the cam belt, but removal literally takes 1 minute, You have a car with one of the finest engines ever produced, so getting your water pump leak sorted and the security of a cam belt replacement will give you much peace of mind.
In my many years of various LS ownerships, the only issue with any engine was a coil pack on an early MK2. Hang on to what you have and look after it, there really is nothing else out there like it, and I believe it ended with the demise of the LS430, and every LS before it shares the same honour!