First we find the volume of the boat.
m*g*0.69V = 50*g
V = 50/(.69 * 1000) =0.072 m^-3
For the boat to be fully submerged the maximum buoyant force (p*g*V) must equal the total weight of boat & sand
p*g*V = (m + 50)*g
m = p*V - 50 = 1000*0.072 - 50 = 22
22 kg is the maximum amount of sand that can be put on before the boat sinks.