Well, there would be many factors to determine whether or not the car would be safe enough.
1) Whether it would float or not - This is dependant on two things, the weight acting downwards and the Buoyancy (force acting upwards). For the car to float the buoyancy would have to be greater than the weight acting downwards. So this means we need to know the weight in order to counteract it. This brings variables such as the amount of people that will be held within the car, the weight of the engine... etc
2) Stabilisation - Ok so the car floats, great, but now comes the question on how to stabilise it. In a straight line it may be fine to use but when turning it could easily flip (as shown in Top gear
) This again would be dependant on things like the severity of the corner you are turning, the COM, the speed at which you are travelling at etc...
If you were to do this then I'd suggest building a car designed for a specific location such as a lake. Quite a vast amount of lakes have speed limits for environmental/tourist reasons so you wouldn't have to worry about a top speed as such due to the fact that it wouldn't be allowed to go that fast anyway! This would also reduce risks I stated before.
Anyway, I hope that helped you in some way
