Analysing the statics, yes. The body of water on the viaduct is a fixed amount between the upstream and downstream lock gates. The mass of the boat is added, the water is displaced, and the level rise is dependant on the area , ie the distance between the gates.
Except that the amount between the upstream and downstream gates isn’t fixed (what happens when the downstream lock is empty and you have to fill it?), and you don’t drop the boat straight into that body of water, it floats in from a lock when the gates are opened, at which point the total mass of water/boat between the locks is unchanged – the water the boat displaces goes into the lock instead.
The only reason adding a boat makes any difference to the water level is because you have to operate the locks. One way to look at this is to consider boats coming from the upper pound via the upper lock. When the lock is emptied into the pound with the aqueduct, the amount of water being transferred is the same whether there are 2 boats in the lock or only 1 (or none, but then you wouldn’t be emptying the lock). Clearly when you open the lower lock gate of the upper lock the water level doesn’t change. When you float the boat(s) out of the lock the water level doesn’t change. Therefore the second boat being in the lock makes no difference at all to the static water pressure on the viaduct compared to there only being one boat in the lock.
Of course the other point is that there is no difference in static pressure on the aqueduct when the boat is on it or when the boat is on another part of the pound which isn’t on the aqueduct.
