What is the procedure if the skipper gets sick?

If the skipper is named in the charter contract (and not a freelance skipper you have hired yourself), the charter company is responsible for them. As such, the charter company agrees to provide you with a skipper who is qualified to sail: in the event of illness or other impediment, the charter company must therefore replace them before departure to ensure the service is provided. You do not need to take any action; the replacement is the charter company’s responsibility.