Setting up a custom roblox studio water physics script is one of those tasks that can make your game feel incredibly polished, even if the math seems a little intimidating at first. While Roblox provides built-in terrain water that works pretty well for most people, it doesn't always cut it if you're trying to build a stylized low-poly game or a specific type of boat simulator. Sometimes you just want a simple Part to act like water, and for that, you need a bit of code to handle how objects float, sink, or bob around.
Why Move Away from Terrain Water?
Roblox's default terrain is cool, but it's a bit of a "one size fits all" solution. If you've ever tried to make a tiny pond or a specific mechanical water trap, you might have noticed that terrain can be clunky to shape. Plus, the physics behavior of terrain water is baked into the engine, meaning you don't have much control over how much "push" the water gives back to an object.
When you write your own script, you're in the driver's seat. You can decide if the water is thick like syrup, bouncy like a trampoline, or just a simple surface that applies a bit of upward force. It also lets you use regular Parts as water, which is a massive win for performance in certain types of builds and allows for way more creative freedom with textures and shaders.
The Basic Logic of Buoyancy
At its heart, a roblox studio water physics script is just a math equation that says: "If an object is below this certain height, push it up." In the real world, this is Archimedes' principle, but we don't need to be scientists to make it work in a game. We just need to apply a force to an object that's stronger than gravity when that object is submerged.
If you just apply a constant upward force, the object will usually go flying into the sky like a rocket. To make it look natural, the force needs to be proportional to how deep the object is. The deeper it goes, the harder the water pushes back. When you balance that out with the object's weight, you get that nice, gentle bobbing motion everyone loves.
Setting Up the Detection Script
Before you can apply forces, your script needs to know when something has actually hit the water. There are a few ways to do this. You could use the .Touched event, but that's notoriously unreliable for physics-heavy tasks because it doesn't fire constantly while an object is sitting still.
A better way is to use a loop—specifically RunService.Heartbeat—to constantly check the position of your floatable objects relative to the water's surface. You can define a Y-level as your "sea level" and then check if Object.Position.Y is less than that value.
Using Raycasting for Accuracy
If your water isn't just a flat plane—maybe you have waves or different heights—you'll want to use raycasting. By casting a ray downward from the object, you can detect exactly where the water surface is. This is the secret sauce for making boats that actually tilt and roll with the waves. It's a bit more intensive on the script, but for a main mechanic, it's worth the extra effort.
Writing the Upward Force Code
Once you know an object is in the water, you need to apply a VectorForce or a LinearVelocity. In the past, developers used BodyForce, but Roblox is moving toward the newer Mover Constraints, so it's a good idea to get used to those.
Here's a simple way to think about the script's logic: 1. Calculate the depth (Surface Height minus Object Height). 2. Multiply that depth by a "Buoyancy" constant. 3. Apply that value as an upward force on the Y-axis.
Pro tip: Don't forget about mass! A giant heavy boulder needs way more force to float than a tiny wooden plank. You can get the mass of a part using part:GetMass() and then multiply your buoyancy force by that number to keep things consistent across different object sizes.
Handling the "Wobble" and Damping
If you just apply upward force, you'll notice your objects start bouncing like crazy. They'll hit the water, fly up, fall back down, and eventually vibrate into orbit. This happens because there's no "drag" or "friction" in our basic math.
In a real roblox studio water physics script, you need to add damping. This basically means you slow the object down based on its current velocity. If the object is moving fast, the script should apply a little bit of counter-force to steady it. This turns that jittery vibration into a smooth, realistic float. It's the difference between a glitchy-looking game and something that feels professional.
Making Players Swim
Water physics isn't just for crates and boats; you probably want your players to interact with it too. By default, characters in Roblox will just walk on the bottom of a part unless you change their HumanoidState.
When your script detects a player has entered your custom water zone, you can use Humanoid:ChangeState(Enum.HumanoidStateType.Swimming). This automatically triggers the swimming animations and changes how the controls work. When they leave the area, just switch them back to FallingDown or GettingUp, and the engine handles the transition back to walking. It's a simple addition that makes the whole system feel integrated into the game world.
Performance Considerations
If you have a hundred crates floating in a lake, running a complex calculation for every single one every frame can get laggy. Optimization is key. One way to handle this is to only run the roblox studio water physics script on the Client (the player's computer) for objects near them, or use a "sleep" state for objects that aren't moving much.
Another trick is to use CollectionService. Tag all your floatable objects with a tag like "Buoyant," and then have one single script that loops through everything with that tag. This is way cleaner and more efficient than putting a script inside every single part in your workspace.
Common Mistakes to Avoid
A big one is forgetting to account for the object's center of mass. If you apply the upward force only to the center of a long boat, it might stay level, but it won't react realistically if a player stands on the front. To fix this, you can put multiple "attachment points" on the corners of your object and calculate the buoyancy for each point separately. This makes the boat tip forward if the weight is in the front, which looks awesome.
Also, watch out for "Force Fighting." If you have other scripts trying to move the object at the same time, they might fight each other and cause laggy, stuttering movement. Always try to keep your physics logic in one place if you can.
Final Touches for Realism
If you really want to go the extra mile, add some sound effects and particles. When an object's velocity is high and it hits the water surface, trigger a "splash" sound and some blue particle emitters. These small visual cues distract the player from any slight imperfections in the physics math and make the whole experience feel much more tactile.
Creating a custom roblox studio water physics script might take a afternoon of tinkering with numbers, but the result is a much more unique game feel. Whether you're making a high-seas adventure or a cozy fishing spot, having control over how things float gives you that extra bit of creative edge. Just remember to keep your math simple, watch your performance, and don't be afraid to tweak those damping values until it feels just right.