Carson,
As mentioned above if your Kenwood HU has a changer DIN (multi-pin) input then this is easily converted to a universal Aux input.
All HU power sounds anemic in the context of an open boat, especially when you begin to move. This will be remedied with external amplifiers. Keep in mind that the 4 times 45 watts is grossly over-stated by every HU manufacturer.
Your existing HU will not last indefinitely but it would be nice to maintain it for a while to free up more budget to improve what you don't have and allow you to step up to a little higher quality. The real expense comes in doing it twice versus doing it right the first time.
Thinking ahead, you will certainly want the convenience of dual zone (tower to in-boat) controls at the driver's helm. Any inexpensive solution (like a Kicker 4-channel zone control/line driver for example) will automatically step up the voltage and make the existing HU preout appear more robust. In this case, if you are mostly using an ipod as your music source, this reduces your HU to not much more than a volume control.

David
Earmark Marine