I created an area in the ocean to force fish to the surface using soul sand so I can collect them as food and the related drops. A few times a drowned mob will bounce around on the water surface in the bubbles and dies. Twice they have dropped tridents. I can not pick them up (bummer!) nor will they despawn after a very long time (hours of real time, many Minecraft day/night cycles).  The tridents just bounce around in the bubble column and stay in the same location.