Thanks everyone for your replies.
After consulting with the local OHS regulator it was found that there is no legal way to risk assess that seatbelts are not required, therefore that part of the argument is gone. Seatbelts are compulsory, end of story. We will be putting a seatbelt cutter and window breaker into every fork instead.

For the issue of preventing a fork going into the water, does anyone have any data that would suggest a speed that the forklift would be stopped by a 8 inch high square edged kerb?
Thanks
Dave