I put XM on my boat a couple of years ago. I have the little Sony removeable one that they quit making. It works great, I'm going through an RF modulator now but that is coming out soon. It definitely sounds better through an aux input.
Mine is picky about the power supply, it takes 6 volts and is noisy without good power. That's a pain.
Plenty of boats here have Sirius, that's what comes on Supras. I'm sure you'll like it. I've listened to both a lot, there's great stuff all around. Plus, it doesn't skip!