Prepaid customers are assets or liabilities for the company

Our Company has started 3 passenger ships. We take advance form our registered agents and recharge their account associated with us. Now the can book tickets directly from that account based on the available balance they had on their account. Since they always pay us in advance, they always show a negative balance under the debtors had in our balance sheet. Can they be created under current liabilities, since their balance in refundable in case we fail to deliver them.