Funny you should mention that -- a friend just forwarded an email sent to her elderly mom from the doc with coronavirus advice. The doc said she'd be happy to write a medical note for anyone who wanted it but to be aware that airlines and cruise lines were ignoring them and that there was a little chance of a refund.
Not surprising, but the wording of the contract controls. If the doctor note meets the terms set in the contract, they don’t have a choice. You may have to go to court, but if they don’t comply, have a lawyer draft a nasty-gram warning them of the risk of litigation over breach of contract, that their practice will be reported to the FTC (Federal Trade Commission) and possible class action suit by ALL the parties who they are refusing to honor their end of the contract.
Also, with a pandemic going on, entire cruises could (and probably should) be cancelled on the basis that the pandemic frustrates performance of the contract. Understandably, they don’t want to lose the income from these trips, but if they are sued for people getting infected and dying because of conditions on the ship, even if they win in court, it will get expensive. There’s no way to sanitize a ship in the time normally allowed for turn around in port.
While I know the useless WHO won’t call this a pandemic for market reasons, they are warning anyone particularly vulnerable to avoid crowds, planes and cruises. It’s pretty much a given that flying and cruising is a bad idea right now in general and for some a definite NO.