I'm in a graphic card driver update lol but you could make all these projectiles get a passenger area_effect_cloud when thrown, then execute all the summoned area_effect_cloud summon drowned if they are not passengers any more...
That's somewhat interresting, I'll work on that... but first, reinstall minecraft lol
If you want to summon drowned with arrows, it would be much simpler, as you could simply execute when in ground, then delete them
If you know there is a contest for winning a minecraft account, I'm interrested!
Otherwise, if you are interrested to buy an account for me in exchange of command/datapack making, I'll be happy to work for you!