diff --git a/api/animate.js b/api/animate.js
index d2973999..de3b8168 100644
--- a/api/animate.js
+++ b/api/animate.js
@@ -15,6 +15,11 @@ const determineDesiredShapeType = (animationName) => {
   return "vertical";
 };
 
+const isOnSurface = (physicsMesh) => {
+  return typeof flock?.checkIfOnSurface === "function" &&
+    flock.checkIfOnSurface(physicsMesh);
+};
+
 const updateCapsuleShapeForAnimation = (physicsMesh, animationName) => {
   if (
     !physicsMesh ||
@@ -28,7 +33,15 @@ const updateCapsuleShapeForAnimation = (physicsMesh, animationName) => {
     return;
   }
 
-  const desiredShapeType = determineDesiredShapeType(animationName);
+  let desiredShapeType = determineDesiredShapeType(animationName);
+
+  // The horizontal capsule is designed for airborne movement. When a Fly
+  // animation is used on a surface (e.g. swimming), keep the vertical capsule
+  // so the character doesn't become unstable on slopes near the water's edge.
+  if (desiredShapeType === "horizontal-fly" && isOnSurface(physicsMesh)) {
+    desiredShapeType = "vertical";
+  }
+
   if (!physicsMesh.metadata) physicsMesh.metadata = {};
 
   if (physicsMesh.metadata.currentPhysicsShapeType === desiredShapeType) {