[Alex W]

There are a lot of DC to DC power supplies available for under $10. I'm using this one for my Netduino-based wind instrument:
Amazon.com: Retailstore 2PCS LM2596 Buck DC-DC Adjustable Step Down Power Supply Module Converter New: Computers & [email protected]@[email protected]@http://ecx.images-amazon.com/images/I/[email protected]@[email protected]@51KgMaHYhnL

It works for me, but there are tons of other options to select from as well.

 

[Alex W]

Simply put, I wanted the input voltage to always be 12v. Sometimes boat batteries can drop below that due to discharge, so I wanted the ability of the Pi to keep running. Those step downs wont work in that situation.

I thought that the Pi could run on 5V, but didn't look closely at it. My Netduino wants 7-10V ideally, so a step-down only power supply was more than adequate.