Minecraft wiki entry for Drowned, associated text: 0.53125% (about 1 out of 188) naturally-spawned drowned drop a trident when killed without Looting, because only 6.25% (1 of 16) drowned spawn with tridents and from these, 8.5% drop the trident. With Looting III, 11.5% of drowned drop the trident, so the overall chance is 0.71875%.