Dakota Johnson's new superhero movie bombed at the box office -- but she's not trippin' ... 'cause the water in Mexico is fine, especially with her boyfriend right next to her.

The actress stripped down to a white halter neck-style swimsuit as she cooled off with her man Chris Martin in the beachy waters of Puerto Vallarta Sunday -- continuing her IDC tour by proving not even a major film flop was gonna ruin her good time.

The couple could be seen recharging on a sunlounger ... before Dakota dried off with a towel after a refreshing dip in the ocean.

Now, some may argue that DJ, perhaps, fled to Mexico to avoid all the 'MW' noise after it tanked this weekend. However, given her unbothered promo tour -- where she even admitted she hadn't watched the film much to the dismay of comic book fans -- something tells us she probably didn't even register how poorly it was received ... by fans and critics alike.

Safe to say ... Dakota didn't really put her best foot forward promoting the film -- at least on the face of it -- and the figures speak for themselves. Whether she's to blame for any of it though ... well, that's anyone's guess.

The running total for the six-day holiday weekend -- beginning Valentine's Day and ending on President's Day -- was just $25.8 million domestically and about the same amount globally ... definitely not up to par for a major superhero flick opening.

In fact, The Hollywood Reporter put it down as the worst opening for a Sony movie featuring Spider-Man-related characters ... and the consensus seems to be the same across the board.

In the meantime, Dakota's living her life and doing her thing ... she's probably already forgotten she was even in this dud.