Agreed the two reasons I can think for turning it off would be the issue of light, unless they were in a rush and he started shutting things off, GPS, radio, transponder etc; dumber things have happened before. After all, they had almost touched down. Or... They simply weren't needed. After all, they had arrived at their destination, so no need for a GPS any more.
Being Marine GPS units they may have been in a cigarette lighter power socket. It could have fallen out. So many explanations, all pathetically small, but when put together, lead to this kind of outcome.