The engine can most likely work both directions equally.

The propeller is optimized to work in forward direction, so it will have worse efficiency going in the other direction.

The rotation would have to be limited when going in reverse because of cavitation (and maybe other structural limitations). That again is a result of the wrong shape of the propeller (when in wrong direction).

The hull will have significantly more drag when in wrong direction.

Now, engine power is defined as whatever it can put out and if the engine works the same way in both directions then power is the same also.

So you can think this way: most likely it has the same power as going forward but it can't use it and whatever it can use will be much less efficiently translated into motion.