I was incorrect. Data. When I switched to the iPhone I lost my unlimited plan. During the sailing season I notice a definite spike in data.
You may want to try using it with airplane mode turned on. If the nav app you're using downloads the chart to the phone, it may be able to use GPS without enabling data. Also it may save your battery over using Bluetooth but I can't say for sure. I don't know what the consumption difference is between BT and GPS.