Debit cards in some geographies are disallowed because of fraudulent signups.
If DigitalOcean accepted your debit card on account, that means they authorized it, verified what card is it and allowed it to be used with you are account. In other words, you should not be in problem.
If your debit card is not allowed, you would instant get message on account creation that Debit cards are not allowed and you would be asked to add another one. So once they authorize it, you are not in problem.
For more information you can always contact support, as they can take closer look into your account. :)

Learn more about it on DigitalOcean Billing FAQ