Went swimming and now my New Apple Ultra will not come back on?

I suspect water has penetrated and therefore the Watch is ruined.

Water resistance is not guaranteed and will only be covered (outside the UK and EU) if you have a paid-for AppleCare+

You ought to reach out to Apple Support directly. You should be able to reach them here  Click here > Contact - Official Apple Support  or phone numbers  Click here > Contact Apple for support and service - Apple Support   or Click here > Genius Bar Reservation and Apple Support Options - Apple 

I know what it says, and yours is possibly that 1 in 10,000 that leaks (my speculation)

Small print will endure.

I never get my Watch wet even though at my advanced years I really should wear it in the shower, but I dare not, especially as it says soap will degrade the seals.