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Kylie Jenner, Tyga

FAMEFLYNET PICTURES

Kylie Jenner is still in celebration mode when it comes to ringing in her 18th birthday!

The E! star was spotted flaunting her bikini bod while wearing a tiny string two-piece during a yacht party with pals in Mexico Wednesday.

Kylie, who continued to rock cornrow braids, was surrounded by a number of her friends, including sister Kendall Jenner, Hailey Baldwin, Pia Mia and rumored boyfriend Tyga.

The group was photographed soaking up the sun and adventurously jumping off the end of the boat to enjoy a dip in the water.

"They took the wave runners from the shore out to the Lolo yacht that was anchored near [Joe Francis' home] Casa Aramara," a source tells E! News of their adventure-filled afternoon. "As they climbed off the WaveRunner, Tyga helped Kylie balance and held her around her waist so that she could get on safely. They climbed aboard with a few friends and headed up to the bow of the boat. They sat on the front as they cruised around and went for a ride. Kylie was laying in between Tyga's legs and resting up against him."

"Everyone was having an amazing time and the best day. At one point, Hailey jumped from the top deck into the ocean. She encouraged Kendall and Kylie to try it, so they took her advice and climbed up to the top. When they were ready, the sisters held hands and then jumped into the ocean together," the source says of the BFFs. "They both screamed on the way down, but they had a blast and big smiles."

E! News broke the news Tuesday that Kylie had jetted off to Mexico for a post-birthday vacation with a small group of pals. Kylie's family members were not supposed to be joining her for the brief trip south of the border, but Kendall joined in on the festivities anyway.

The rumored couple arrived on Monday evening to Puerto Vallarta with a group of friends. They were chauffeured 45 minutes away to Punta Mita to stay at Francis' luxury beachfront home.

Kylie Jenner, Tyga

FAMEFLYNET PICTURES

The house has 12 bedrooms and rents for $12,000 per night, a source tells E! News. It comes with a private chef, butlers and masseuses.

Yeah, it's basically a dream land.

According to the insider, the group had a welcome dinner prepared by a private chef at the home and then a small party where they all hung out on the patio by the pool.

Kylie and Tyga then spent some one-on-one time together on the beaches on Tuesday morning. The pair rode a WaveRunner together, and Kylie had her arms wrapped around Tyga's waist and was holding on tight, an eyewitness tells us.

"When Tyga slowed down, Kylie rested her head on his back and they both admired the view. They were checking out some giant pelicans in the water and taking everything in as they rode around. Then they went back up to the house and spent some time laying out by the pool."

It looks like the birthday vacay is off to a successful start!

New episodes of Keeping Up With the Kardashians return later this year only on E!

PHOTOS: Kylie Jenner and Tyga's cutest pics!