Yeah afraid of water is nothing.. again a training issue.
The tail swishing is a symptom of something. My horse never swishes his tail (except for flies).. he's very happy and in good manners. When he does, I know something is wrong. With a horse that does it all of the time, I'd be concerned.
It would be more fair to yourself, and to the horse, if you got back into riding before you bought her/him. And you'd have a lot more fun too, you'd understand how to correct bad behavior and have confidence.
Owning a pony when you're younger is a lot different than a horse when you're older.
Get as much education/instruction as you can before diving into horse ownership.
"Strength is the ability to use a muscle without tension"