I swim about a mile a couple of times a week.
For getting in and out of my chair, I park my wheelchair by the steps. If I'm at my normal pool, I grab a couple of kick boards and aim for those getting out of the chair and then again when getting out of the pool. These are thick and provide protection on my butt. If I'm at a hotel pool, I just use towels. I've practiced getting in and out of my chair enough so that it's not an issue.
When swimming, I usually use a snorkel, it just makes it a bit easier. I also made something to hold my legs together and provide a bit of flotation. I took a noodle and cut two 4 - 6 inch pieces. I then ran a stretchy shoelace between them. I put that around my legs just above my knees.