Wow - well, shame on Prestige Travel for booking you to leave May 16 from MSP to a sailing in England on May 17 at 4:30pm. All sorts of things could have happened, let alone the oxygen mask issue.

So, your question - is United liable? No, they aren't. If anything, Prestige Travel is. I'd expect the E+ to refunded and a small goodwill travel cert - but you should take this up with Prestige Travel - that is where I'd start.