Assuming this works

src/main/java/net/citizensnpcs/npc/entity/CitizensHumanNPC.java

@@ -138,10 +138,12 @@ public class CitizensHumanNPC extends CitizensNPC implements Equipable {
     public void update() {
         super.update();
         if (isSpawned() && getBukkitEntity().getLocation().getChunk().isLoaded()) {
-            mcEntity.move(0, -0.2, 0);
+            if (NMS.inWater(mcEntity)) {
+                mcEntity.motY += 0.08F;
+            } else
+                mcEntity.move(0, -0.2, 0);
             // gravity! also works around an entity.onGround not updating issue
             // (onGround is normally updated by the client)
-            NMS.trySwim(mcEntity, 0.16F);
         }
     }
 }

src/main/java/net/citizensnpcs/util/NMS.java

@@ -208,7 +208,7 @@ public class NMS {
     }
 
     public static void trySwim(EntityLiving handle, float power) {
-        if ((handle.I() || handle.J()) && Math.random() < 0.8F) {
+        if (inWater(handle) && Math.random() < 0.8F) {
             handle.motY += power;
         }
     }
@@ -276,4 +276,8 @@ public class NMS {
             Messaging.logTr(Messages.ERROR_GETTING_ID_MAPPING, e.getMessage());
         }
     }
+
+    public static boolean inWater(EntityLiving mcEntity) {
+        return mcEntity.I() || mcEntity.J();
+    }
 }