Not sure about Lowrance/Navionics, however I bought a second power cord for my Humminbird and attached it to an old 12 volt power supply. I believe it was an old phone charger. My Humminbird needs something like 430milli amps of power to run, the charger puts out like 3 times that. I don't remember the exact numbers, it was over a year ago I hooked it up, many beers since then.

Anyway, I sit in my recliner drinking said cold one and fire up the sonar unit. Pull up the map and can mark spots all day long. Honestly, though, the best option is Autochart Pro. Record all day and then scan the SI on your pc looking for new cover/structure from your recliner. Let's me focus on fishing boat navigation while on the water.

Hopefully you can get a second power cord and bring the sonar unit inside to mark points.