Removing a block from fluids / water.json or fluids / lava.json removes the mechanics of the associated fluid, but adding a different block from the default blocks in these files does not give all the mechanics associated with the fluid.

For example (all my tags have "replace":true)
1st step: remove the "minecraft:water" block from the water.json tag removes the mechanics of the block water (including swimming).
2nd step: to add the block "minecraft:air" in this same tag gives no mechanics of the fluid "water".
3rd step: add the block "minecraft:water" in the tag lava.json gives the block of water the texture of the lava, but no other mechanics -> the water always behaves like air block.
4th step: add the "minecraft:lava" block in the tag water.json and remove the "minecraft:lava" block from the lava.json -> lava texture changes but not mechanics.

Please excuse me for my English, it's not my main language.