I once had the same query, trucks being used on a dockside handling boats into the water. Together with an UK H&S Officer, we agreed that if a risk assessment suggested that the hazard of falling into the water exceeded the risk of overturning, with the consequent danger of being trapped in the cab of a sinking truck, it was better to agree that, when operating close to the quay, it was permissible to not wear belts, provided that, elsewhere, where there was no danger of falling onto the water, belts were used.
The only problem then is policing the policy.