Yes, just a sheet of glass. a vinyl or aluminum will window will be between 2 1/2 to 3 1/4 thick , the glass will be as thick as the glass, I'd go 3/16 minimum to 1/4 on that. Direct will give the most options if you want anything decorative. id go direct set if it was my house.

I don't know the answer to your question, but I understand wanting the window back. I once lived in a house with a window in the shower and it was lovely to take a morning shower while looking outside. However, before you go to the effort, imagine yourself showering there. Would you be exposed to anyone via the window? It looks like the shower is on the back of the house, but are there neighbors? Also, water pooling on the window sill and running down the inside of the wall can be a problem. That may be why the previous owner put the shower liner in. So it might not be as simple as "cut a hole and caulk".

I agree with Cpartist"s layout and like moving the closet over as suggested by D M so that bedroom 3 has easier access to the bathroom. Do you have room to have the toilet face the tub and add a linen cabinet next to the toilet? How wide is the tub toilet area?

old three deckers all over boston built in early 1900s all have same layout, windows over old tub and all are problematic. my solution, buy a nice white mini blind, put in the window and then cover the wall up - outside you just see a window with the blind down, inside nice shower no risk, no headaches. on rentals you always have to plan for the stupidest people possible. Renters dont care if water builds up and window rot till they fall out. add ceiling fan that goes on automatically when light in shower is on.. ventilation solved