Options for solving the stereo control issues:
1. Connect an RF remote using a Y cable. The one listed in the link below floats, Or there are stationary marine grade remotes available, also uses Y connector. RF remote cost is in the $90 + Y $30 = $120. Price is based on eBay, if you don't shop around you'll pay 2x that.
2. Use this Bluetooth device.
- my experience has been that streaming Bluetooth audio doesn't have as good (it's good but not great) as a wired connected device. That said, this might be the way I solve it for my boat. (Cost $60 + Y cable = $90)
3. In some cases I hear if you aim the remote at the blue part of the glove box, it works. (Free)
I also thought about using an IR extender, but it cost as much as the other options and isn't clean.