Get your feet wet. Walk boat out as bit and only sit down when it is fully floating. Easier if you can sit your butt all the way down and then fold your legs in (as opposed to if you can't fold your legs in, so have to sit on the back deck, which is much less stable, and slide in). Landing is reverse.
You could use your paddle as a brace, but I never liked how that seems to be hard on the paddle.