If you're in uncharted waters, you are in a situation that is unfamiliar to you, that you have no experience of and don't know what might happen.
('Unchartered waters' is an incorrect form that is a common mistake.)
If a ship sails under a flag of convenience, it is registered in a country where taxes, etc, are lower than in the country it comes from, so if someone does something under a flag of convenience, they attempt to avoid regulations and taxes by a similar means.
(USA) It means until the very last possible moment or until every possibility is exausted:
You boys always stay until the last dog is shotI will stay until the last dog is shot to complete this project by deadline
(Expression my mom who was born in 1917 in Wisconson always used.)
If you up the ante, you increase the importance or value of something, especially where there's an element of risk as the term comes from gambling, where it means to increase the stake (the amount of money bet).