The Triferris Cotswold Super Sprint triathlon is a great early season open water swim event. Note must go to the organisers, Triferris, who have a wealth of knowledge on how to run a great event.
The open water lake is beginner friendly. The event also draws some top age group talent so caters well for all level of competitor.
I've also uploaded a video on the event to my YouTube channel at: http://bit.ly/2TriferrisCotswoldTri