I don't have too much helpful experience here - as I just don't show my anxieties around my family because they are a huge cause of anxiety for me!

For the most part it would be easy to not talk about the water issue with them. But I suppose if they notice that you're not showering that it makes it hard to avoid.

So instead, I'll offer some (probably weird but it's where my brain is right now) alternatives to having an actual shower if the actual shower scares you. Can you fill the tub up, even just a tiny bit, to use it like a bucket? And then wash yourself with a washcloth and soap. For your hair, you can just use the sink. There are also "dry" shampoos that you can use in between when you actually wash your hair.

At least then you'll be clean, so the rest of the family will just have to stfu in regards to anything like that.

That's absolutely horrible that they would hold your head underwater That's so traumatic and... violent. I'm so sorry that you have had to experience that.