The interesting data they provide is how they move and the air currents that cause that motion.
Amateur science is still science and woe betide the free people that moves towards reserving science to professionals disproportionate to risk of harm.
Pretty low. Successfully crafting it, getting it off the ground, and having it go as far as the ocean without popping is a pretty good accomplishment providing value disproportionate to cost.
More environmental damage was likely done fabricating the plastics in the balloon than crashing it into the sea.
I spend most of my days picking up trash from the ocean (<1 mile away from me). Suffice to say, our perceptions of the difficulty and nobility of throwing trash into it are vastly different.
Amateur science is still science and woe betide the free people that moves towards reserving science to professionals disproportionate to risk of harm.