lens - What lenses should I choose for a night boat cruise?

I'm going to a boat cruise tonight in London. With the following kit to choose from:

• Nikon D7000


• Nikkor 35mm f/1.8


• Nikkor 16-85mm f/3.5-5.6


• Nikon SB-400


• Yongnuo YN-465

I will definitely have both lenses with me, but I don't really like switching between the two too much. Do you think it'll be more useful to use the 35mm (with or without flash), or use the zoom lens and rely on flash all the time? The fact that the boat is moving all the time also means that, when I see something interesting, I need to be ready to take the shot straight away - the scene will have totally changed by the time I manage to change a lens.

Majority of the photos I will be taking will probably be of people, both indoors and out on the deck against the cityscape. But there will still be plenty of opportunities for pure cityscape photos as well, e.g. the house of parliaments, tower bridge, the few skyscrapers.

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_{This was originally part of another question on photo-SE.}