Phimai is a great place to see longboat racing. Every year during the Phimai festival in November there is longboat racing. This weekend (11-12 October) saw 2 days of longboat racing in gra-bueang-yai (กระเบื้องใหญ่), a small village about 4 miles from Phimai town.
Next month will see the Phimai festival in Phimai town. Longboat racing is scheduled for 6-9 November in the usual location, Lam Jakarat, with access from Anantachinda Road or nearby. Good views of the racing can be had from the bridge on Anantachinda Road. For details of the 2014 Phimai festival see here: