I just added two more to my tank the other day and they floated for about a day and then sank! I think mine were floating because they were relatively dry from shipping but once they were saturated they dropped!
I read an article about marimo balls from Lake Akan, Japan — supposedly, the origin of the "true" marimo? — that stated "This algae is free floating, and displays unique behaviors in response to habitat changes. They will float or sink in the water in response to columns of light, and will actually roll around on the bottom of the lake (aquarium)."
Marimo balls will float due to O2 adhering to the surface of the ball. It's actually a good sign that your aquarium is healthy. I've deal with the floating balls by attaching them to fine polyester thread (preferred over nylon) that magicians use in their acts. Achor with a Stainless Steel nut (1/2" is fine) and it must be 316 stainless to minimize leaching in the water. When initially purchased, your balls might not float so make sure they are in floating condition by leaving them in the aquarium (photosynthesizing) before attachment. The effect is quite "other worldly" with green balls suspended in the aquarium. See photos.