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.
That is a relatively fundamental result. With current materials, it's pretty much impossible to levitate anything against Earth's magnetic field. You can't put enough current through a wire to create a force bigger than its own gravity without using unreasonable amounts of power. Even if you put in that power, you'd melt the wire.
If you actually want to accelerate it upward to any meaningful speed, you'd need even more power.
Really it may be conservative to say another 100,000 tonnes for all the stuff other than the 200,000 tonnes of the ship itself.