Rhys and Sienna plan to wed

She may have taken her time before deciding to marry Rhys Ifans. But it seems Sienna Miller isn't wasting any time when it comes to choosing where to exchange her vows.

The couple have their hearts set on hiring a private beach resort for a week on the island of Isla Mujeres, eight miles off the coast of Cancun, Mexico.

A source said: "This is the perfect location for Sienna to have her big day. This particular resort is literally in the middle of nowhere, so the only people to witness their wedding will be close friends and family.

Sienna Miller and fiance Rhys Ifans get sun kissed in Mexico yesterday

"They fell in love with Mexico the first time they went there - so it makes sense for her to choose it as a location for her wedding."

The pair have always seemed rather an unlikely pairing. Yet pretty actress Sienna Miller and her unkempt boyfriend Rhys Ifans certainly seem to have a lot in common.

Soaking up the sun, drinking cheap beer and taking an occasional dip in the azure waters of the Gulf of Mexico from their private launch seemed to be their common cause yesterday.

Feeling the heat: Sienna makes a splash in the water with Rhys by her side

Cool down: Sienna gets a helping hand back onto the boat

Miss Miller, 26, a pioneer of the boho-chic look, looked to have rubbed off some of her style on the Notting Hill actor who wore an bandana, sarong and aviator-style sunglasses as they relaxed on deck.

Though Mr Ifans certainly did not have the same sleek physique as his younger lover.

Miss Miller wore the briefest of bikinis which she almost lost completely when lifted from the surf after a dip by the boat's skipper.

The actress gets a much-needed hosing down

A happy Sienna looks as if she has no care in the world as she soaks up the sunny conditions in Mexico

The private resort where they plan to wed, complete with a large ten bedroom villa, boat ramp and several guest houses is believed to be owned by a close pal who she met on set of a recent film.

Due to its location, much of the food for the wedding is being shipped in from Cancun's mainland. After the ceremony, guests will then be taken back to the villa to feast on traditional Mexican dishes and knock back Mexico's best tequilas.

The couple became fond of the Latin American city during their romantic holiday last year.

And it seems it remains their first choice as the couple were spotted frolicking on the beach only.

Sienna takes some time out to check on her tanned legs

Rhys, who at 39, is 13-years Sienna's senior, first asked her to marry him in August when he sent the Factory Girl star a cryptic message in Welsh, which read "Marry the Misfit".

He proposed again in December - and although Sienna's publicist has denied reports of an engagement - Sienna finally said yes earlier this month.

The loved-up pair are also looking for a home in Oxford as well as a cottage in Wales, in a bid to remain completely private.

Sienna and Rhys were back in LA today, where Si is filming GI Joe with costars Dennis Quaid and Channing Tatum.

The couple relax on board as they take in the surrounds and are no doubt imagining their upcoming wedding here

In the film, she plays the villainous Baroness - a femme fatale skilled in espionage. The actress is rumoured to have gained 5lb and has started intensive fitness training in preparation for the role.

Excited Sienna has said: "This is definitely not an independent film — this is big-budget stuff... I finally get to really kick some butt.

"I've started taking boxing lessons, and I've also had to learn how to fire guns with live ammunition.

"I'm a baddie, running around kicking everyone while wearing tight, thigh-high boots. And I'm getting muscles everywhere."

Meanwhile, Rhys has decided to put acting on the backburner once he completes The Boat That Rocked, to re-launch his music career. After fronting the Super Furry Animals in the 1990's, his main focus is to launch his new band The Peth.