Which do you think bothers Bethenny Frankel's soon-to-be ex-husband more -- seeing her amazing body in a tiny bikini ... or that she's rocking it with her new man?

Frankel spent the day of New Year's Eve on the beach in Miami with new beau Michael Cerussi. Bethenny's daughter with estranged hubby Jason Hoppy is reportedly with the couple on the trip.

As if all this wasn't enough to irk Hoppy -- whose divorce from Bethenny is still not quite final -- Frankel tweeted, "NYE day & i'm exactly where i want to be with exactly who I want to be with."

Ouch.