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.
More environmental damage was likely done fabricating the plastics in the balloon than crashing it into the sea.