You pack out the height issues with things like this.
http://www.screwfix.com/p/broadfix-precision-wedgepacker-medium-77-x-8-x-45mm-pack-of-200/76521
Or even better used wooden laths under the 18mm ply. To get ideas of the packing height, if tiles silicone polish floor or if flags lay polythene over the top. Then put daubs of plaster onto the floor in the low bits, place on 18mm board and tap and bash till level. Wait till plaster sets, lift up and you have your heights of the daubs you cut your laths to (or packers). Or place concrete daubs on low places and place 18mm board on top and use as set, or even better, as seen here numerous times, lift up the flags (for reuse later) and lay concrete base to place frame on. Done.
If you say steel beams running front to back, probably with intervening walls supporting the steels as well and having walls on top, I suspect will be strong enough.
A large bath full to top can easily come in at 250Kg and you don't read too many stories of baths falling through floors.
In my experience (and having seen on this forum) failure occurs in couple of ways.
- Catastrophic tank failure. Side panel cracking and falling out, due to tank no longer being on a flat base. Stand (usually from Ikea) is completely unsuited for the weight and just collapses.
- Side panel cracking due to tank not being on a flat base.
- Silicone tearing in opposite corners due to tank not being on a flat base.
The tank not being on a flat base has occurred due to either floor giving way (usually floor boards) or the stand deforming in one corner.