Stig,
Thanks for your ideas. Unfortunately (the) newspapers weren't always exact when it came down to dates, so when the incident has happened isn't all too clear.
I gathered that on 4 July 1918 RAF Felixstowe N4513 was lost and another damaged after a skirmish with German airplanes from Zeebrugge. No Dutch involvement here..!
See e.g.
https://livesofthefirstworldwar.org/lifestory/6831030 and scroll down to appropriate date.
As with the big RAF/German air combat on 4 June 1918, the story in my thread doesn't fit with regard to fate of the aircraft.
I checked online Dutch newspaper archives and the story first appeared on 3 July 1918 and was repeated with the very same words in several other papers on 4 July 1918.
First date reportedly when the fishing vessel ('Neutraal') had returned in her home port of IJmuiden, Holland and crew had given their account.
The incident with the 'water plane' most likely therefore not more than a week before up to 3 July, assuming that under the wartime conditions the fishermen stayed in home waters and wouldn't be out for a lengthy period.
Regards,
Leendert