If you've ever spent time developing in Studio, you know that getting a roblox swim script to actually feel fluid is way harder than it looks. Most people just stick with the default Roblox water physics because, let's be honest, they're convenient. But if you're trying to build something specific—like a deep-sea exploration game or a competitive swimming simulator—the default "float and kick" mechanic starts to feel a bit clunky and restrictive.
The problem is that the built-in system is designed to be a "one size fits all" solution. It works for a casual dip in a terrain-based pond, but it doesn't give you much control over speed, buoyancy, or those cool underwater animations that make a game feel polished. If you want your players to feel like they're actually slicing through the water rather than just jittering on the surface, you've got to get your hands dirty with some custom Lua.
Why the default swimming logic feels off
Let's talk about the "jellyfish" effect. You know what I mean—when a character enters the water and suddenly their physics change in a way that feels totally disconnected from their ground movement. The default roblox swim script logic uses a specific humanoid state, but it doesn't always transition smoothly.
One of the biggest gripes developers have is the lack of momentum. In real life (and in high-end games), you don't just stop dead the second you stop kicking. There's a glide. There's a sense of weight. To get that in Roblox, you usually have to bypass the automatic state changes and tell the engine exactly how you want the character to behave when their hitbox overlaps with a water volume.
Getting the basic script structure ready
Before you start typing away, you need to decide where this script is going to live. Usually, for something that affects how the player moves and what they see, a LocalScript inside StarterCharacterScripts is your best bet. This ensures the code runs specifically for the player's character and can react instantly to their input without waiting for a round-trip to the server, which would make the swimming feel laggy.
The core of any decent swim system is detecting where the player is. You can do this by checking the Humanoid.FloorMaterial (though that doesn't always work for water) or, more reliably, by using Humanoid:GetState().
Handling state changes
You'll want to keep a close eye on Enum.HumanoidStateType.Swimming. When the character hits this state, your script should "wake up." Here's the thing: you can actually disable the default swimming state if you want total control, but that's often more trouble than it's worth for beginners. Instead, it's usually better to let Roblox handle the "is in water" detection and then use your script to override the forces applied to the character.
I like to use a loop—usually connected to RunService.RenderStepped—that checks if the player is currently submerged. If they are, you can start applying custom LinearVelocity or VectorForce to make the movement feel more intentional.
Adding those smooth animations
A roblox swim script is basically invisible if you don't have the visuals to back it up. We've all seen those games where the character is clearly "swimming" but their legs are doing a walking animation. It looks terrible.
To fix this, you need to load a custom animation track onto the Humanoid. When the script detects the swimming state, you play the animation; when they leave the water, you stop it. But here's a pro tip: use fading. Don't just snap the animation on and off. Use AnimationTrack:Play(0.5) to let the swimming pose blend in over half a second. It makes the transition from walking to diving look ten times more professional.
Also, consider different animations for different directions. If a player is swimming upward, their body should tilt. If they're idle in the water, they should be treading water, not just frozen in a diving pose. These small details are what separate a "basic" script from something that players actually enjoy using.
Physics and movement tweaks
This is where the real magic happens. If you want a "fast" swim or a "scuba" feel, you need to play around with the character's density and gravity. By default, characters in Roblox have a certain amount of buoyancy, but you can override this by adjusting the CustomPhysicalProperties of the character's limbs.
I've found that the most satisfying swimming mechanics often involve a bit of "drift." When the player lets go of the "W" key, don't just set their velocity to zero. Instead, use a lerp (linear interpolation) function to gradually slow them down. This gives the water a sense of thickness and resistance.
Another trick is to change the Fov (Field of View) slightly when the player enters the water. A subtle zoom or a slight wideness can simulate that feeling of being submerged, especially if you pair it with a blue-tinted color correction effect.
Dealing with part-based water vs terrain
Here is where a lot of people get stuck. Roblox has two types of water: the built-in Terrain water and "fake" water made from transparent parts.
If you're using Terrain water, your roblox swim script can rely on the built-in Swimming state. But if you're making a stylized game with part-based water, the Humanoid won't know it's supposed to swim. In that case, you have to write a "Zone" script.
You can use the Touched event, but it's notoriously glitchy for water. A better way is to use WorldRoot:GetPartBoundsInBox or the newer Spatial Query API. Basically, every frame (or every few frames), the script checks if the player's torso is inside a part labeled "Water." If it is, you manually set the Humanoid state to swimming or use a BodyVelocity object to let them "fly" through the part as if it were liquid.
Final testing and bug fixing
Once you've got the logic down, you're going to run into bugs. It's just part of the process. One common issue is the "infinite jump" glitch where players can jump out of the water and literally fly into the sky because the script thinks they're still in a low-gravity swimming state.
To prevent this, always make sure your script has a "clean up" function. When the state changes to Landed or Running, you need to instantly kill any forces you've applied and reset the gravity.
Another thing to watch out for is the camera. Swimming often involves looking up and down to change depth. If your script doesn't account for the camera's look vector, the player might find themselves stuck moving horizontally even when they're looking straight at the bottom of the ocean. You want to map the movement direction to the camera's orientation so that "forward" always means "where I am looking."
Wrapping things up
Building a custom roblox swim script isn't just about making a character move through blue blocks; it's about creating an atmosphere. It takes a lot of trial and error to get the damping, the speed, and the animations just right. Don't be afraid to tweak the numbers—sometimes a 5% change in movement speed makes the difference between a game that feels "floaty" and one that feels "weighty."
Take it one step at a time. Start with the state detection, move on to the movement forces, and then polish it off with animations and camera effects. Before you know it, you'll have a swimming system that feels way better than anything the default engine provides. Happy scripting!