The Spiderwick Chronicles: Wyrm King Book 8 by Tony DiTerlizzi

Nick and Laurie thought their worries were behind them when they drove all the giants into the sea. But now, the Grace kids have come to tell them they may have more trouble coming their way! It turns out the giants control the population of Hydra, a dragon-like creature that is creating sinkholes all over Florida. But with the mermaids refusing to return the giants to the shore, the nixies still missing and the threat of a destroyed Florida drawing closer, the kids have to take matters in their own hands.