A waterproofing membrane
is vital; I understand it is a rental but all the more reason to ensure the tub surround or the shower enclosure - including around the window - is watertight. Depends on where you're at now; if you're going to demo anyway, then a membrane is better than the surround. Ditto if it's just the window wall. But the window itself can be waterproofed but it will involves doing the tiles as well so you might just want to look at a complete job.
IMO well worth the time and effort. Nothing like having to redo a shower after a few years -tenant there or not.