Thanks for the comments all.
An astronomical telescope, seems most likely. Still what where the guys looking at by daytime in a horizontal direction. Furthermore they were looking land-inward down the new waterway.
Would such a telescope capture the whole waterway? And, yes there was something like a computer attached to it. So they might make some still images with that?
By the way, there's no second telescope on the right. That's a bench.
I'm not too worried about the place being shut down. It's a public place open to all, and above all, this is the Netherlands, you would not encounter any problems taking pictures easily. Though I can imagine you get to be send away near enclosed terrains by security guards.