Its common practice for tilers to use a heavy ply on bathroom floors, tongue and grove is there to stop drafts more than anything, it isnt any stronger than plain old timbers layed flat
As long as its sealed it will work okish, If you dont want toungue and groove though I would just stick with a plained redwood PSE timber in lengths as its easier to work with than ply and will resist moisture better as it wont delaminate at the edges
As you said, bathroom grade laminate is going down, this is usually sealed with a glue not just a click lock system and will have the under lay and membrane, so If you ever need to take it up you are going to have to rip that out and then replace it anyway
Another tip, dont buy timber from the DIY chains, its whitewood and its not very good quality