A cheap option would be to insert an SD memory card into your unit at the console. Your waypoints & such would be saved on it. Then take it out & insert it in the other unit when you are using it. Anything you do there will also be saved to the card. Ferrying the card between units is certainly not an ideal solution, especially if you were to drop the SD card overboard, but would only set you back 10 or 15 bucks while you decide if the network port adapter would be worth the price.
(You could post a "want to buy" ad in the classified section of the forum & maybe find a used adapter for much less.)