Since the forces on the ball will be balanced (ie. we know the buoyant force will act upwards, but this must be balanced by the tension acting downwards, or else the cord would snap and the ball would float up) this means the tension in the rope is equal to the buoyancy force. The buoyant force acting on the object is equal to the weight of water displaced - note that the buoyancy force is only related to weight of water displaced, not depth, so the pool depth and cord length are irrelevant.
The net buoyancy force will be equal to the weight of water displaced minus the weight of the ball.
We use the formula: FB = Vg(ρwater - ρball) where V is the volume (of water displaced, equal to the ball's volume), g = 9.81m/s2, ρ is density (ρwater = 1000kg/m3, ρball = 0.3g/cc = 300kg/m3). Vgρwater gives the weight of water displaced (buoyancy force) and Vgρball gives the weight of the ball.
Volume of the ball/water displaced is the volume of a sphere, and the radius is 1/2 diameter, so 23cm or r = 0.23m
V = 4/3 * pi * r3 = 4/3 * pi * 0.233 = 0.050965 m3
Now from the formula for net buoyancy force, FB = 0.050965 * 9.81 * (1000 - 300) = 349.98N
Since the tension must equal the net buoyancy force for the ball to remain stationary, T = 350N