Not since the jacuzzi at Plato’s Retreat has Manhattan seen an aquatic playground like Spa Castle Premier 57. When we checked out the beloved Queens spa’s seven-month-old Midtown offshoot a little while back, there was no shortage of cavortage in the hydrotherapy pool. So what if the backsplash from everyone’s water-jet massages was landing in our champers flute — we’re all friends here. But one thing did put a damper on this waterworld: there was no rooftop pool a la College Park location.