Recent Wikipedia rabbit holes have taken me to some interesting places. Did you know you can cook watermelon rinds?!? Apparently it tastes like a cross between squash/zucchini, carrot and cucumber. I don't normally even buy watermelon but I have to try this out...
Melon & Lemon jam was a staple of my childhood. That is, jam made from melon rind cooked translucent with lemon juice for the pectin. Very beautiful on the plate or as a topping on pies - 1 cm cubed pale yellowgreen jewels in a sweet/tart matrix. Gorgeous on crumpets. Not made anymore as farmers aren't growing the right kind of melon for jam, which was too bitter and stringy to eat raw but the rind cooked up beautiful. Let us know if watermelon rind cooks up translucent - if so I want to try making jam with it.
Part of my wiki travels suggest that the suspected ancestor of sweet watermelons was indeed bitter and stringy as well. A close relative continued to be grown for jams (due to its high pectin content) as citron melon. I imagine it's quite a niche product.
This recipe for watermelon rind jam - no watermelon flesh - sounds very similar to what you were describing. Looks translucent!
e: it appears there are in fact many receipes for watermelon rind preserves (cubed) - most stating they are old recipes, some from the Depression era, or traditional recipes hailing from Iran, Armenia, Greece... I reckon if you get a cheaper watermelon that's got more rind than usual it would be great for making these preserves.
Banana peels gross me out, the texture is yuck. But watermelon rinds sound intriguing - so much firm edible matter that gets wasted, it's clean and fresh and blemish free (it's generally recommended to peel the tough green skin off). I'd give it a go