Hello Jean,
Sounds like an interesting project and I wish you luck. I cannot comment on numbers of ship Spotters/Photographers in the world, but as far as the UK is concerned, ship spotting is very much a minority activity and at a guess it's probably only a few thousand out a population of 60million? If you check Facebook for shipping groups you can see how many members each group has, you could also contact the World Ship Society to ask how many members they have. I still belong to the Thames Ship Society (although I am no longer anywhere near the Thames!) and I think they have around 300 members so numbers are not great. There are many reasons why numbers so small but two main reasons are (1) fewer places to see ships as most ports are now in isolated locations away from most people, and (2) security...even if you live near a port the ships are now hidden away behind fences and security guards!!
I think perhaps any economic benefits from ship spotters are minimal (unless you have a hotel or B&B overlooking a busy river!!) so I would be inclined not to mention us but instead mention general tourism and health benefits of being able to walk across bridges, and of course general tourists will take photographs of ships if they see one passing.
I'm sorry if this sounds negative reply, I would be happy to give you further thoughts if you think it may help.
Andrew
ps. Sorry, just a paranoid thought, but if you are publishing details of bridges where ships can be seen passing then you may attract attention of the security services, it's the world we now live in I am afraid, in the UK we have laws that prevent you from having information "of use to terrorists".....whatever that means!