I use a bluetooth gateway to stream Pandora and Spotify directly from my iPhone to the aux port on the radio. That way you can keep your iPhone in a sealed waterproof (and floating!) enclosure while playing your music...
Blackberry bluetooth stereo gateway
Then use this to hardwire to your auxiliary switched power source. It comes with a mini-USB cord which is what the Blackberry device requires.
Arkon direct wire USB power
Whole installation takes less than 30 minutes and minimal skills. I have one of these in each of my vehicles and the Bennington. Should pair and work with any bluetooth enabled A2DP phone. That includes the iPhone 3GS, iPhone 4 and iPhone 4S and most newer Android devices.
Of course this won't keep your iPhone charged but I've never run out of battery on a typical lake day. But if you think that's an issue then you can add this to the list (or just take your iPhone out of the sealed bag and charge normally via a standard 12V socket USB charger).
Mophie Juice Pack Case