I had some time to play around with the mechanics for the drowned mobs in 1.13, this design will lure them into a bubble trap than shoots them up then pushes them to there death, it has the same mechanics as how you would make zombie/skeleton spawners but its made to work with this type of NPC.




BUILDING TIPS:

• Find a location where ocean ruins are, drowned will respawn at those locations as of 1.13 release, the closer you are to the ruins the more efficient the grinder will be.
  

• The farm will only work at night so its best to make an AFK room where you can stay safe over the nigher so the manta ray does not come and kill you in numbers.
  

• You will need one villager at near the top of the ocean this seems to work the best in this update when night comes drowned will come closer to the surface and see the villager that is just under the water.
  

• you will need soul sand before you can build this in survival as drowned don't float up so you will need to push them up with soul sand. if you are having problems making a portal you can just use water and lava sources to build a portal, this way you don't need to find diamonds first.
  

• last but not least the drop will need to be 23 blocks I suggest going 25 blocks just to be safe but drowned have 20 harts on easy and they are immune to 3 block drops hence the extra 3 blocks you will have to measure the distance based on difficulty if you play on normal or hard.