Why didn't the magical water just deliver the heart to Te Fiti instead of Moana?

Just toss the damned thing on the island that Te Fiti was on. No need to get Moana involved.

Is an actual human, with tiny human dexterous fingers, needed to place the heart back in place?

Is Te Fiti distrustful of anyone other than a "special" human that will melt away her hate and suspicion, and that the water knew that Moana was "the One" who would do that?

Then why the need for Maui? And why can't the water just directly propel her directly and uneventfully to the island?

Is it that Moana needs to have her mind appropriately "conditioned" in order for the "moment of empathy" to occur between her and Te Fiti before the trust required for the two to come together can happen? The water knew all that, huh? Oh please, now the writers are just pressing their luck.

I feel like the writers started with a premise where the solution was a little too simple, and so convolutions were added for plot, but it's artificial and unconvincing.

That's what happens when you create a story by filling in the blanks in a mad-libs, Hero's Journey template.

The movie is a thumbs up because of the sheer visual beauty and the technical aspects, but boy does the story leave something to be desired.