I don't know Christchurch but if you didn't antifoul in Keyhaven and left a boat in the water for a month you'd have a fair growth, especially during the summer months. To give you some idea of how quickly this stuff grows, if you leave your tender on a mooring for a week as often do when going out sailing, at the end of the week there is already a thick coat of green slime.
It would probably come off with a power washer but it would slow you down a bit. Hilda Maud stays on her mooring all year and is antifouled but even with antifoul enough stuff grows within a month to knock a knot or so off her top speed.
I'd guess that Christchurch isn't that different.