It sounds like you have tried a lot of things. One thing you may not have tried is DEEP WATER aerobics. You do not have to know how to swim. Your hair barely gets wet! In a class with a lifeguard present, you wear a floatation belt mid body, and go to the deep end. The pool should be about 80 or 81 degrees, otherwise you will get too hot. Many pools are overheated for kiddy lessons, arthritis and so on. But some YMCA's or clubs will have a pool with colder water for lap swimmers and deep water aerobics or a form of water Tai Chi, called Ai Chi. (Ai Chi is done in deep water with belt, but is often done in warmer water as it is not aerobid and is very slow moving, great for stretching and range of motion.)
Balance is not an issue in Deep water aerobics classes, as you are not touching the bottom, and the floatation belt keeps you upright. However, you do need to have enough strength to keep yourself upright as possible, with your tailbone pointing down to the floor of the pool. Deep water workouts have a lot of core work, as that is what is helping you stay upright. Find a class at a YMCA or similar place, as it is hard to know what to do once you're in the deep water with your belt, unless an instructor is there.
The class will have water walking, jogging, a move called cross country skiing, which is a long striding motion with pretty straight arms and legs...and other stuff where you use your hands, with or without dumbbell type floats or webbed gloves. This helps strengthen uppper body. There are even ab exercises you can do in the class.
People who have not done deep water aerobics, look at classes and think that no one is doing much, because there is not much to see....it's all underwater! The water supports the body, but it also creates a lot of resistance to push against and this is why it is a good workout with no impact on your joints, while allowing a greater range of motion than the same thing on land.
You can make the movements smaller for less resistance, open your fingers instead of having a scooping hand, and can go at a slower pace than the count the instructor might be on.
But it is quite strenuous, and I would recommend going slowly and carefully the first few times, tell instructor you are new, have MS (if they know what that is, but not all will know anything about it), have balance problems, overheating problems, etc. Bring water and keep it on the side of the pool.
While exercise is good for everyone and is often touted for people with MS, I realize that finding MS adaptations for effective, safe and fun exercise is very limited. Don't let people tell you that exercise is the answer to all your MS problems. But it can help, if you find something you can do and enjoy doing.
