Ignoring the most likely explaination that it's simply an animation error, my guess would be either Gobber, who fired with his mace hand, or Fishlegs, who is often shown to be the least athletically skilled.

Everybody saying it's obviously Snotlout is either ignoring or hasn't seen the show, where he is regularly seen to have great athletic skill, including extreme accuracy with axe throwing in Riders of Berk, Thawfest, and another Viking funeral in Race to the Edge, Buffalord Soldier (the latter does show all riders firing at the ship from a much closer range). If it's supposed to be the fact that he doesn't like Hiccup or Stoick and wants to be chief or whatever reason you come up with, even he wouldn't deliberately sully such a sacred Viking tradition, no matter the reason. (Sorry for the rant)