My good friend's four year old drowned & died in the bath while he was getting her sister in her PJ's (gone maybe 3 minutes...he and his wife were both in the house...neither more than a room away) the year my son was born...so I'm paranoid...maybe ESPECIALLY because my son is a swimmer and really comfortable in the water. I also know of too many kids (from working at a hospital) who slipped and hit their heads and drowned (some made it some didn't). They were twitching so it sounded like they were fine.
Now...I've been drowned myself (3 times...as a training exercise)...and if you catch it fast enough drowning is something that is TOTALLY recoverable (although it can lead to a tendancy to catch pneumonia). While I know exactly what to do if someone's drowned (although I don't have a crash cart in my house...something I've always wanted) I'm still very very paranoid about it.
Last year when my son was 6, I started leaving the room for a few minutes here and there...but I'd still talk to him while I was gone (if you can talk, you can breath...did you know that? If someone's shouting they're drowning, they're not. As soon as water starts filling your lungs there's a paralytic response...hence part of why waterboarding is so scary). This year (7) I'm leaving him for longer periods...but about half the time I still just sit with a book and read. In pools/rivers/oceans though I keep my eyes on him ... and if there's current I stay within 6 feet. But in a bath I feel I can have my attention on my laptop or in a book.
Nudity isn't an issue in our family...and we've got a few more years before masturbating will be...so for now, I just do as I feel comfortable with. Even though I'm fairly sure he'd be fine, like I said, I'm paranoid.