I leave DS2 (10 months)alone, very briefly, the water is shallow, he's sitting (& standing well), we have non-slip bath matts. I leave him long enough to grab his clothes, the phone, a towel, a nappy, etc. He is not going to come to harm in the 10-15 seconds it takes to do those things.
Once DS1 was about 8 months old I used to put his baby bath in front of the open door to the bathroom, where I could always see him, & run around upstairs doing jobs. You think me reckless but the water was A) shallow, B) had a non-slip mat in it, and C) (most important), he spent most of his time climbing in and out of the baby bath. I wouldn't have done that with DD or DS2, though, DD was so quiet I never know what she's up to, and DS2 was very likely to topple over.
I used to leave DD & DS1 briefly like Coldtea describes, but keep close tabs listening to what they are doing, easy enough with DS1 (by then well over 3 and a constant chatterbox).
Don't worry much about leaving DD alone now (3.5yo).