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.
