blub

    private ArrayList<Player> waterbreathingenabled = new ArrayList<>();
    private HashMap<Player, Inventory> inventoryHashMap = new HashMap<>();
    private ItemStack itemon = new ItemStack(Material.getMaterial(getConfig().getString("ITEMON")));
    private ItemStack itemoff = new ItemStack(Material.getMaterial(getConfig().getString("ITEMOFF")));
    private ItemStack openItem = new ItemStack(Material.getMaterial(getConfig().getString("OPENITEM")));

    @Override
    public void onDisable() {
        super.onDisable();
    }

    @Override
    public void onEnable() {
        ItemMeta meta = itemon.getItemMeta();
        meta.setDisplayName("§aEffect enabled");
        itemon.setItemMeta(meta);
        ItemMeta meta1 = itemoff.getItemMeta();
        meta1.setDisplayName("§4Effect off");
        itemoff.setItemMeta(meta1);
    }

    @EventHandler
    public void PlayerJoin(PlayerJoinEvent event) {
        waterbreathingenabled.add(event.getPlayer());
        inventoryHashMap.put(event.getPlayer(), Bukkit.createInventory(null, 9,ChatColor.translateAlternateColorCodes('&', this.getConfig().getString("Gui-Options.Title"))));
        Inventory inventory = inventoryHashMap.get(event.getPlayer());
        inventory.setItem(0, itemon);
    }

    @EventHandler
    public void PlayerLeave(PlayerQuitEvent event) {
        waterbreathingenabled.remove(event.getPlayer());
    }

    @EventHandler
    public void PlayerOpenInv(PlayerInteractEvent event) {
        if (event.getItem() == openItem) {
            event.getPlayer().openInventory(inventoryHashMap.get(event.getPlayer()));
        }
    }

    @EventHandler
    public void PlayerClick(InventoryClickEvent ev) {
        if(ev.getInventory() != null) {
            if (ev.getInventory() == inventoryHashMap.get((Player) ev.getWhoClicked())) {
                if (ev.getSlot() == 0) {
                    if (waterbreathingenabled.contains((Player) ev.getWhoClicked())){
                        inventoryHashMap.get((Player) ev.getWhoClicked()).setItem(0, itemoff);
                    } else {
                        inventoryHashMap.get((Player) ev.getWhoClicked()).setItem(0, itemon);
                    }
                }
            }
        }
    }