The bug
Normally spawn eggs can be right-clicked in water to spawn mobs, even when you're in the water yourself. However, I tried right-clicking bubble columns to spawn the mobs inside of them, but it didn't work.
I tested this with waterlogged blocks too, or just blocks like sea grass. Right clicking the blocks themselves works like normal, but right clicking the water the block itself is in doesn't work.
Affected blocks
General waterlogged blocks(E.g. slabs, trapdoors, glow lichens... etc.)
Kelp
Kelp plant
Seagrass
Tall seagrass
Bubble column
Confirmed for 1.13-pre7