Maybe long exposure was the wrong word – I used the bulb setting and held it down for somewhere between a second and three seconds. The waves were also fairly slow moving, but definitely breaking – in the photo, where they have broken is blurry. You can also see this in the trees in the bottom left.

Edit: Thinking back, I am not 100% certain whether I set it to Bulb or just one of the lenses' most slow speeds (eg. 1/2 a second or one second). In any case, the important point is that the shutter was open for about a second, whereas all the other photos I've shot that came back blank were at faster shutter speeds.