things that will introduce unwanted behaviour with adding a fluid to the water tag:

can pathfind over/through fluid
corals plants and blocks !-> dead coral plants and blocks
plants will be hydrated near fluid
concrete powder -> concrete with fluid
conduit waterlogging
farmland moisture near to fluid
cobblestone and obsidian generation with lava + fluid
whether kelp can generate in the fluid
magma + fluid fizz
respawn anchor explosion resistance uses water block resistance
whether seagrass can generate in the fluid
fluid makes it possible to pathfind slabs
whether sponges can remove the fluid
whether snow can spread (over? the fluid)
whether sugarcane can survive next to the fluid
whether tall seagrass can survive and generate in the fluid
whether the player gets water vision when in the fluid
whether the air bubbles render on the HUD when in the fluid
whether bubble column(up) particles can exist in the fluid
whether bubble column(down) particles can exist in the fluid
whether bubble particles can exist in the fluid 
something to do with fog rendering(honestly not sure) - i think maybe water fog
whether the water overlay should render if the player is in the fluid
some water debugging thing. i dont think its really too important
whether a dispenser can dispense boats in the fluid
whether a dispenser can use bottles to grab water bottles from the fluid
drowning in the fluid
something something pathfinding
swim tasks will pass the conditions with the fluid
whether dolphins will jump in the fluid
whether the find water goal can find the "water"
panic goal will look for the fluid
swim goal will pass conditions with the fluid
boats. i mean most things to do with boats tbh
whether experience orbs do their special underwater movement stuff in the fluid
something to do with falling blocks. not really sure possible the fact that it wont place the block when it falls in the fluid
whether items do their special underwater movement in the fluid
whether drowneds can spawn in the fluid
whether endermen teleport in the fluid
whether guardians can spawn in the fluid(and whether they can pathfind??)
whether witches will equip a water breathing potion in the fluid
whether zombies will convert to drowneds in the fluid
whether dolphins can spawn in the fluid
whether squids will flee towards the fluid
whether foxes can swim in the fluid??
whether dolphins can swim to items in the fluid
whether turtles will go towards the fluid
whether fish will move in the fluid
whether skeleton horses will play a water ambient sound instead of their normal ambient sound in the fluid
player food exhaustion modifier whilst swimming in the fluid
player dig speed whilst submerged in the fluid
whether the turtle helmet will provide water breathing in the fluid
the `PlayerEntity#updateIsUnderwater` will be true in the fluid
whether fishing bobbers do their thing in the fluid
whether the fluid can survive in ultra warm environments (the nether for example)
whether using a glass bottle on the fluid will give you a water bottle
whether `IWorldReader#isWaterAt` will return true
whether world carvers can replace the fluid
whether `RandomPatchFeature`s can replace the fluid
whether or not `SphereReplaceFeature`s can replace the fluid
height of ocean ruin pieces will continue if the fluid is present
whether spawners can spawn entities in the fluid(maybe, i think?)
some forge thing to do with rendering air whilst in the fluid. (im not really sure)