Never take the last fence for granted - It happens sometimes when riders are clear to this point then push on to the last fence pushing their horses out of balance and then they can leave a leg and land in a heap on the other side - all the hard work wiped out in one mistake.
The jump into the water had a big drop, you needed to have your weight equally in your stirrups and really use your core, therefore being in a balance for part b in the water. This should have been 4 or 5 strides on a curve.
The approach is through dark woods so you have to be aware of the change of light. You need a coffin canter with plenty of power this will enable them to not over jump the log and land too far down the slope.