I just love these questions that deal with nakedness. I wasn't raised in the US and find it surprising again and again what a big deal is made about the naked human body...
I agree with others that say, if neither of you feel uncomfortable showering together, nothing wrong with it!
You obviously feel uncomfortable, otherwise you wouldn't tell him to look away. That really is not a good message you are sending. In my mind you are telling him that he is not to look at a naked female body, implying that there is something wrong with it, even if you don't mean to.
If you don't feel comfortable showering with him, rather don't do it at all, than teaching him that it's bad to look at mommy (or any naked female in his mind) - that's just my opinion.
If your home is not safe for the kids to occupy themselves for 15 minutes, I guess you will have to lock them into a safe room. I personally just turn on the TV for my two year old (she doesn't get to watch a lot of TV, so she will be glued to it, while it's on) and leave the bathroom door ajar so she can come in if she needs to. Our home is babyproofed, so I don't worry about her finding something else to do, if she decides not to watch.