First of all, I find the term "prescriptive" to be rather equivocatory, used primarily to express disapproval rather than to communicate precise meaning, and it quite often just means "more strict than I think is appropriate". To the extent that "prescriptive" has a clear meaning, I disagree with your application of the word to definitions. There are prescriptive and descriptive approaches to writing a dictionary, but the definitions themselves are descriptive. For instance, there was a flap about a dictionary that included in its definition of the word "gay" that one meaning is "stupid". A girl objected to that, and started advocating that the dictionary remove that meaning of the word. So, one person might say "The word is sometimes used to mean 'stupid', and dictionaries are should describe how words are used, so we should include that meaning". That's a descriptive approach to definitions. The girl, on the other hand, was saying "This meaning is offensive, and dictionaries shouldn't offend people, so this meaning should be removed". This is a prescriptive approach. But both "gay mean means homosexual" and "gay means homosexual or stupid" are descriptive statements.

You need a prescriptive, subjective definition of a thing that will transport you over water.

If you want something that will transport you over water, that's not “prescriptive”, “subjective”, or even a “definition”. It's a specification. You aren't saying “things that can't transport me over water shouldn't be called boats”, you're saying “The genie shouldn't give me something that can't transport me over water”. You don't need a new definition of “boat” to communicate that, you just need to phrase your wish as being more specific than just a “boat”. If you really want to have a term that refers to a thing that will transport you over water, you can make up a new word, and give it that definition. If you define a word as meaning “a thing that can transport PhilGoetz over water”, then that will be an objective definition.