Originally Posted by smrobs
The first thing that pops into my head is that you probably stayed astride of her and sitting like you would normally ride on dry ground. Generally speaking, horses are good swimmers, but they can only carry so much and keep their head above water. Think of it like this, a horse trying to swim that is carrying a rider in normal riding position is about like a person trying to swim with a big brick in their arms above their head. Because of the laws of buoyancy, more weight above the water automatically makes their body go down farther in the water...and if they aren't fit enough to compensate for the added weight pushing down on them, then they will not be able to keep their head above water.
The biggest problem with that is it can be exceptionally dangerous, especially for someone who isn't terribly experienced with doing it properly. It would be very easy to get one of your own legs kicked and end up with a broken bone or.....she could panic and start looking to you to keep her safe and end up drowning you by swimming/pawing over the top of you.
I walked into the water slowly next to her (didn't even have to go in front and pull her or something) and when she got scared and turned back to the shore I kept my arms on her (holding mane on her whithers) and my legs near the water surface. So that I could control her head and catch the reins if needed but keeping my body away from her legs. When I was much younger and inexperienced a horse decided to swim across the pond and as I was trying to stop him, I got kicked in my leg... I let the horse go (and he just swam across and started grazing on the shore) and luckily I wasn't hurt, but it made me think about where my body is and where the horses legs can be while swimming.
Still, what you say is a good point for everybody.