This is a popular question. My suggestion is a small class D amp (energy efficient, 12V, which is a consideration on a boat) such as the one linked below for $79. It has Bluetooth and will drive a subwoofer when you decide to add one. Then you connect your phone via Bluetooth and use any service such as Pandora that you like. You will need an external switch to select inside or outside speakers.
If you insist on a radio (I personally think it is useless) then you can look into car audio units which will give you more features such as radio, CD, fading between front and rear speakers but the problem with those is that 1) they consume more power and 2) fiddling with the settings, while convenient when stuck in traffic in a car, is a distraction on a rocking boat. I ditched my car unit for the Fosi and have been very happy with it. For example, you listen to a tune in the car, get to the boat, flip one switch and the music continues. Zero hassle. Before, I would need to press a button for a couple of seconds to turn the unit on, then connect Bluetooth, then adjust the sound between front and rear (it took 30 key presses to switch from full back to full front), guests on board had difficulty using it because it was too complicated to operate or connect their phones to it.
Lastly, the subwoofer, anyway you look at it, if you want to have good sound, you need to add a subwoofer. I added mine in the cabin. I also have the TV connected to it so that when I watch a movie I get a good 2.1 sound that rocks the boat. I think 2.1 is enough for the boat. You may be tempted to go full surround and some boats have it but then it becomes quite complex, you need a surround sound amp and the space in the cabin is difficult for surround sound.
If you insist on SiriusXM you can go that route as well (a car unit will be best in this case). Definitely stay away from marine units, they are not worth it in my opinion. Marine speakers are worth it because the plastics fades slower and you can upgrade your speakers later if you so desire.
SV Pizzazz