Inconclusive. Plenty of [i]tried[/i], but the details about their success or failures are left vague. It's not to say that it's impossible, as there is certainly those who have tried. But allying with a Siren comes with the same risks as there always is about allying with the fey, and even you'd know that most tales of greedy mortals working with equally greedy fey never end well. It's only fairy tales where pure and kind people do a favor for the fey and they get showered with niceties, but again those are fairy tales. Heck even the stories about pirates joining up with fey tend to also be part of those same fairy tales.

In conclusion: Most of the stories you've heard makes it seem like a bad idea, but mostly because those are children stories. Actual tales of pirate and fey interactions almost never go into an alliance, and often are sources of conflicts, albeit occasionally profitable ones.