Those speed bumps get you every time!
A run along the foreshore usually gives me around 40m of climb. Seems dodgy to me too.
A few years ago Rob Preston had some incredible climb on his Garmin - and he was paddling. Just picked up the arm movements over and over and over again!
Commercial GPS are notoriously bad with climb. The most accurate models have data points with an error of up to 3m. Since you move quite far laterally, the errors tend to average out reasonably well, but even on a really hilly course your climb is quite small compared to the possible errors.....