The bug
If you look at a block while under the water, then under the 'Targeted Blocks' section it will appear despite the fact there is also a 'Targeted Fluids' section and the 'Targeted Blocks' section should say whatever non-fluid block I am looking at.
Linked issues
is duplicated by 5
MC-130586
F3 menu shows minecraft:water and minecraft:lava in both "Targeted Block" and "Targeted Fluid" categories
Resolved
MC-131035
Targeted Block shows mincraft:water when looking at underwater blocks
Resolved
MC-131362
While under the sea, the F3 menus's "Targeted Block" line displays water despite other block is targeted
Resolved
MC-131827
Targeted Block does not ignore liquids
Resolved
MC-131934
"Targeted Block" in F3 screen doesn't work properly inside fluids
Resolved
Can confirm for 1.13-pre4.
This issue affects lava as well.