When creating superflat world, lakes cannot be removed if decoration is present. This issue has been reported few times since 1.13. I just confirm it is still an issue in latest version 1.14.2.
MC-153273
Lakes cannot be removed in superflat world
Resolved
Duplicate
0
Duplicate of MC-125173.
In the future, please leave a comment on the already existing ticket to let us know that the issue still exists, instead of creating a new ticket.