Driving rain and choppy seas made navigation tough, but the 22-year-old seemed unfazed as he rounded the first mark four body-lengths clear of his nearest challenger.
However, he was soon caught and a five-strong lead group quickly formed, gradually pulling away from the field.
By the halfway mark, pre-race favourite Yasunari Hirai had worked his way to the front and, as the leaders approached the final mark, the Japanese Olympian looked to be the man to beat. As they rounded the last marker, Hirai and Capp were neck and neck, with New Zealand Open Water champion Radford and series leader Philip Ryan hot on their heels.
As the quartet hit the beach it was Capp who was quickest, sprinting across the sand to claim victory by five seconds. Hirai edged Radford for second.
Meanwhile, Charlotte Webby (New Plymouth) is also $5000 better off after claiming the overall series victory in the women's event. Webby had to settle for second behind Emma Robinson (Wellington) on Saturday but still took out the series.
- 2015 State King of the Bays results: Men: Nathan Capp (Tauranga) 29:54, Yasunari Hirai (Japan) 29:59, Kane Radford (Rotorua) 30:00. Women: Emma Robinson (Wellington) 31:59, Charlotte Webby (New Plymouth) 31:59, Stefannie Gillespie (Dunedin) 33:09.