Could someone advise please!

Rennovating a bathroom.....had to remove plasterboard wall (fully tiled) of shower cubicle. Also had to remove a ledge above the bath - also made of plasterboard and then fully tiled.

Plasterbaord is so easy to work with - and I have lots of it left - is it really bad to replace the missing bit with plasterboard. All the surfaces will be fully tiled, grouted and selaed as usual - or should I use Marine ply/moisture resistant MDF or something else.

To be honest, I hate using circular saws, jigsaws as I seem to be incapable of cutting straight lines.....plasterboard is just so easy!!!!