Within the same world, you get put in the water at the right coordinates.

From another world, you get put in the nearest air block above the X, Z coordinate.

Re-sending the command will put you in the proper place.

Apologies for the delay in looking into this, but could you clarify what teleport safety settings you're using so I can try and replicate this?

I can reproduce this on the following versions:

[16:49:22 INFO]: CONSOLE issued server command: /ess version
[16:49:22 INFO]: Server version: 1.13.2-R0.1-SNAPSHOT git-Paper-487 (MC: 1.13.2)
[16:49:22 INFO]: EssentialsX version: 2.16-pre1.14
[16:49:22 INFO]: LuckPerms version: 4.3.58
[16:49:22 INFO]: Vault version: 1.7.1-b91
[16:49:22 INFO]: EssentialsXProtect version: 2.16-pre1.14
[16:49:22 INFO]: EssentialsXSpawn version: 2.16-pre1.14
[16:49:22 INFO]: EssentialsXChat version: 2.16-pre1.14
[16:49:22 INFO]: EssentialsXGeoIP version: 2.16-pre1.14
[16:49:22 INFO]: EssentialsXAntiBuild version: 2.16-pre1.14
[16:49:22 INFO]: EssentialsXXMPP version: 2.16-pre1.14

and the following teleport-safety settings in config.yml:

# If the teleport destination is unsafe, should players be teleported to the nearest safe location?
# If this is set to true, Essentials will attempt to teleport players close to the intended destination.
# If this is set to false, attempted teleports to unsafe locations will be cancelled with a warning.
teleport-safety: true

# This forcefully disables teleport safety checks without a warning if attempting to teleport to unsafe locations.
# teleport-safety and this option need to be set to true to force teleportation to dangerous locations.
force-disable-teleport-safety: false

This has now been fixed by #2520/74d96ce - you can download the latest version from the build server.

You can now use the is-water-safe: true/false option in config.yml to control whether water is considered "safe" (can be teleported into) or unsafe (EssentialsX will find a safe location).