Answer:
The towers height is 322m
Explanation:
We can assume that a simple pendulum hangs from the top of the tower.
In this case, the length of the pendulum will be the same as the height of the tower.
We will use the next expression then:

Where T is the period, L is the pendulum length and g is the gravity 
From the previous expression we will have then that the length will be:

Replacing:

The towers height will be then: 322m
The equation for power is P=w/t, where w=work done in joules and t=time in seconds.
in this case:
p=w/t
p=3500/25
p=140
so, the boat was using 140 watts of power.