You may run into legal trouble if you don't label the book "For Fish Only".
Other than that, I'd suggest adding at least 2 more thrashers in. As far as 5 mana creatures go, those little guys are really really good. Water Elemental is fairly dominant when it's in play, so if you haven't tried using it yet then I suggest giving it a shot. It forces the enemy to react, giving you time to support it and cast some smaller creatures (like the thrashers, for example.)
Otherwise, this looks like a good start. People are still figuring out the siren, so let us know what works and what doesn't in your games!