But what of utility?

This introduces the human element.  What will you do with the boiling water?  How much boiling water do you want?  How much boil do you want the water to have? Etc, etc. 

These are questions relating to subjectivity, or opinion. 

If you don't want boiling water, you may not take the time or expend the energy to boil it.

Whether or not you want boiling water is up to you.  HOW you boil water is not up to you - the conditions necessary to boil water are established as unchanging fact in the physcial world of reality.  You decide whether or not you will boil it, if you have the means.

Now substitute the words "morality" and "ethics" for principle and utility.

If we can find a principle of human behavior, much as we found the conditions necessary to result in boiling water, we will have found an unvarying rule of human behavior which exists objectively in nature.

Such a principle will always hold.  It will be predictable - and anyone can apply it, with a predictable and consistent result.  There will be nothing magic about it.  It will not depend upon the existence or edict of a church or government.

No human being ever "invents" a principle - he or she merely discovers it.

When we introduce the concept of ethics, we are now addressing the question of utility and opinion.

Having observed the moral rule, we still must apply our subjective judgement concerning its utility or desirability, and whether and how we will use the principle.