Draught
Roblox gives buoyancy to terrain water and nothing else. A pool, a canal, a moat or a flooded basement built out of Parts does nothing at all: crates fall through it and boats sit on the bottom.
What you get
Draught makes a Part behave like water. Things float at the depth their density says they should, not at a depth you tuned by hand.
How it holds up
A water volume of 120 x 14 x 60 studs with its surface at y = 27.00 holds a crate of density 0.70 with 1.20 studs dry. Drag, and the Swimming state for characters, come with it.
What it does not do
Your Parts keep their shape, material and colour. It gives Part water the behaviour terrain water already has — it does not replace terrain water, and the shape terrain water cannot make is exactly what it is for.
The listing, word for word
Terrain Water cannot fill every pool, canal, moat or flooded room your map needs. Draught makes an upright, non-collidable box Part behave like a water volume.
Objects float from their density and displaced volume instead of a hand-tuned hover height. In the measured Studio proof, three 10-stud cubes settled within 0.25 studs of the waterlines predicted from their densities; a denser cube sank to the floor.
Quadratic drag is included. Humanoid Swimming support is best-effort, not a replacement for Roblox Terrain Water.
Scope is explicit: the water volume must be upright, anchored and non-collidable; other anchored Parts are ignored; tested floaters can take more than six seconds to settle with the default drag. Your Parts keep their shape, material and colour.
Use Draught when you need box-shaped Part water with measured buoyancy and accept those constraints. No network dependency is required at runtime.