On Wednesday, coconut water maker Vita Coco announced it had signed New York Yankees third baseman Alex Rodriguez to an endorsement deal.

Source: Vita Coco

The company sent a picture to the press of Rodriguez holding the product, which did $40 million in business in 2010, with the goal of hitting $100 million in sales this year.

One problem: The folks at Zico, Vita Coco's biggest competitor, which is going head-to-head in a battle to become the leader in the growing $200 million space, say A-Rod is an investor in their company.

Mark Rampolla, Zico's CEO, told CNBC that he was surprised when he saw news of A-Rod's latest partnership.

"Alex has been an investor for a couple years," Rampolla said. "And it's not chump change either. It's a hell of a lot more money than I ever invested in the company."

A news release issued by Vita Coco quoted A-Rod as saying that his trainer suggested the product to him.

"I drink it every day," Rodriguez said, in a statement. "The stuff really works."

"Alex came to us looking to do something this year because he's such a big fan of the brand," Vita Coco CEO Michael Kirban said via e-mail. Kirban acknowledged that he wasn't aware of A-Rod's past involvement with other beverage companies.

So what happened with A-Rod's investment in a competitor?

"I don't discuss my personal investments," Rodriguez told CNBC, in statement read by his business manager, Guy Oseary. "What I will say is that since Zico changed their formula to concentrate, I felt that the taste and functionality was compromised and that's when I started to only drink Vita Coco. I love the taste and purity of Vita Coco."

Rampolla says he's confused.

"We've been sending Alex our product for months," Rampolla said. "We've sent product to his house, locker room, spring training and he has requested it when he's traveling. Not only do we have video of Alex drinking our product from concentrate, but we have him recommending it. Our product from concentrate has the same nutritional value and has the same profile and, according to Nielsen, it is the number one coconut water SKU in New York."

Rampolla, who said Zico's last shipment to A-Rod was last month, then made the slugger an offer.

"If he truly feels that he made a better choice, we'll be more than happy to return his money."

Making the connection to Vita Coco isn't hard. Oseary, who A-Rod met through Madonna, is an investor in Vita Coco as is A-Rod's former girlfriend.

Due to his new contract, Vita Coco's Kirban said that A-Rod "can't, nor does he want to, promote any other beverage brands."

Getting athletes involved in the coconut water space is the next big step. Vita Coco announced Wednesday it also signed Boston Red Sox second baseman Dustin Pedroia. Rodriguez will be paired with Rihanna in the New York market, as part of the company's multi-million dollar advertising campaign.

Athletes involved in Zico include Kevin Garnett, Amar'e Stoudamire, Rip Hamilton and Ray Rice.

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