When splashing through water, wither skull projectiles and dragon fireballs, as well as fireballs and small fireballs use the Friendly Creatures sound category, rather than Hostile Creatures, as to be expected (as they originate from hostile mobs).

This can be tested by flying above water in creative mode, turning only the Friendly Creatures sound category on, and using the commands /summon dragon_fireball ~ ~ ~ {Motion:[0.0,-1.0,0.0]} or /summon wither_skull ~ ~ ~ {Motion:[0.0,-1.0,0.0]} or /summon fireball ~ ~ ~ {Motion:[0.0,-1.0,0.0]} or /summon small_fireball ~ ~ ~ {Motion:[0.0,-1.0,0.0]}{}.