I did this with Mr Chew's ashes in February, from the Nieuw Statendam. The staff who helped me were wonderful, very caring. We had a small group to see him off. It was shortly after sunset, right over the wake.
They cannot tell you in advance exactly when you can do this. It depends on the weather & location, in other words, someone on the bridge. We had about 6 hours notice, which was plenty. When the GS officer phoned me, she told me I could pick anytime that day between 5 pm and 6:30 am. I asked if anyone ever did pick 3 am, or some other time during the late night/early morning, and she said yes! (I did NOT pick that time, Mr Chew & I are neither night owls or early birds, but I imagine those hours might have special meaning for some.)
I did have to show them a death certificate and the contract from the funeral home showing the biodegradable urn. (I also had to show those to TSA when I flew to FLL)