One has to be physically stable when performing daily prayers, and this is determined by ‘urf (what is commonly known). If one was on a boat, how can he determine whether the unstableness is fine or not?
The condition of stability applies in normal situations. If one is on a boat or another type of transport where there is vigorous movement, but he has no other alternative (e.g. one knows that for sure he has enough time to pray after landing, or can stop somewhere on his way etc.), then its fine.
Its also possible to pray while seated to reduce instability.
Answered by: Sheikh Mansour Leghaei