When summoning a shulker riding an entity in air the shulker does not align with the block grid which is very useful in map making, but when inside water the shulker sticks to the grid.

Command used:

summon armor_stand ~.5 ~1.5 ~.5 {NoGravity:1b,Passengers:[{id:"minecraft:shulker",OnGround:1b,NoGravity:1b,Silent:1b,Invulnerable:1b,Glowing:1b,PersistenceRequired:1b,NoAI:1b,AttachFace:0b,Color:0b,ActiveEffects:[{Id:14b,Amplifier:0b,Duration:999999,ShowParticles:0b}]}]}