Lower back exercises in water

I have to work out in water. I have a genetic disorder called mhe that has caused my skeleton to grow wrong. So my ankle, knee and hip joints are out of alignment on both legs. About ever major bone you can think off has something wrong with it or tumors on it.

In short, I have issues with arthritis because my joints are deteriorating. My muscles get antagonized by exercise because they are being forced to work in ways they aren't mean to.

When I was younger it wasn't as bad. I took belly dance lessons, lost weight, started walking.

Go to the point were I couldn't walk very far with out my ankles inflaming, had other issues... gained the weight back.

This year I joined the YMCA with my aunt because she wanted to loose the weight she gained while battling cancer. This October I finally decided I was fed up with my weight and changed my eating habits close to what I did the first time I lost 40 lbs.

So even though it's not comfortable. There are even some moments were it hurts so bad I have to stifle a scream else I alarm the life guards. But it's doable. So I'm going 4 to 6 times a week. I started stretching and 'weight' training and then doing an hour of aerobics. So I get a good work out on my leg muscles, my arms and my abs but I don't know how that affects my lower back.

I've always heard that your abs and your back work as a team. While I'm not ever going to have or worry about tone abs. I do worry that I might be getting them stronger than my lower back muscles.

So is this something I should be concerned about? And if it is, does anyone have any suggestions on exercises I could do that won't drown me. I'm not a strong swimmer