Maelstrom by Anne McCaffrey

In McCaffrey and Scarborough's action-packed, kid-friendly sequel to 2005's Changelings, Murel and Ronan Shongili, telepathic 10-year-old twins with the ability to change into river seals, rescue the inhabitants of the environmentally degraded planet Halau and bring them to their home world of Petaybee. The sentient planet's adaptation to the refugees provides dangerous adventures for the twins, their friends and their parents, geneticist/selkie Dr. Sean Shongili and former Company officer Yana Maddock-Shongili, coadministrators of Petaybee, who discover an aquatic alien race. Meanwhile, off-planet, the Intergalactic Enterprises Company arrests their friend Marmion, who helped transport the refugees from Halau. A cliffhanger ending, ecological lessons, anthropomorphic animals and simplistic good vs. evil morality will speak to middle-school readers but not adults.