Amazingly, that's a severe underestimation.
The force on a wire carrying current in a magnetic field is given by F = B*I*l, and to move it upwards that force needs to be at least equal to gravity, so equate to F = m*g.
- Let's be generous and assume B = 100 uT = 100 * 10^-6 T.
- Likewise, assume the ship is a rectangular box with sides of 400 m x 100 m, so a circumference of 1000 m, and that we can wrap a wire 100,000 times around it, to give total wire length l = 10^8 m.
- m = 200,000 metric tonne = 2 * 10^8 kg (we assume magic wire that is massless).
- g = 10 m/s^2.
Solving for I yields I = mg/Bl = 2 * 10^5 A. That's a lot of current.
If we assume the wire has a diameter of 10 cm (which is ridiculously high considering we just wrapped it 100,000 times around the ship, but whatever) and is made out of copper, it has a resistance of ~200 Ohms. Necessary power to generate such a current is P = I^2*R = 8*10^12 W (= 8000 TW). That's about half the total energy consumption of humanity.
The largest nuclear power plant puts out about 8000 MW, so you'd need a 1000 of them.