I go to the beach all year round - I'm a complete surf addict. The water is definitely getting much warmer now, but doesn't really warm up enough for me to lose my wetsuit until the end of Jan/beginning of Feb.
Good snorkelling spots include Cathedral Cove in the Coromandel, and around Goat Island near Warkworth. If you dive then the Tutakakas/Poor Knights are amazing, and it's worth doing a dive to Rainbow Warrior too.
Water sports are big here - most of my friends were shocked that I'd never waterskiied before, as this is something they have all grown up doing. I have tried, fallen over and I prefer wakeboarding - soemthing else I'd never tried!
I've lived in Cornwall, and I prefer the NZ beaches - there's something about the ruggedness of the west coast beaches that I really love.