The Park Theater is going under the sea for its first show in nearly three decades.“The Little Mermaid” makes its premiere tonight at 7:30 p.m. as the first performance in the restored downtown venue. Other showtimes are Saturday at 2:30 and 7:30 p.m., and Sunday at 2:30 p.m.Director Logan Taylor says there’s been nearly four months of practice leading up to this weekend’s shows. Before Park Theater was available, practices were held at WCMS.“This is a full-fledged musical with singing, dancing and acting,” said Taylor.