Val makes some excellent points. In fact, as I was singing your poem to the tunes of The Water Is Wide and Somos El Barco, semi-submersible, I was actually modifying the tune -- but I didn't know how to indicate that here in the forum.

Keeping the basic structure of a tune while substantially reworking the melody is actually something very commonly done both in Jazz and in today's pop and "r & b" music.   (I actually find it very annoying a lot of the time, especially when the original meldoy is outstanding and the modulations leave that melody both borderline recognizable and far less memorable.
A case in point is the song "Over The Rainbow." It seems hardly anyone who records it nowadays actually sings Harold Arlen's tune; they just sing something that uses the basic phrasing, and if what they were singing were played instrumentally I'm not sure people would know what song it was but I digress.)

Anyway, what Val suggests is probably the best way to go. Start with something like TWIW and change it up to "make it your own."

Or you could borrow melody lines from different songs and piece them together to make a new melody.

I hope you'll find a way to share your song with us when it's finished.

Genie