Frig me I've just spent 20 mins posting something up only to have it dissopear again on me. Now the short version.
Over 40 years of boating, yet to have a flat battery or need to flip the switch over to my second battery. Fishing at night I use the waterproof led flood lights which seem to last for several nights and charge in under two hours.
Even though I do remote trips in the top section of Australia island hoping I now no longer bother with dual battery setups.
My Nav lights are run by rechargeable battery's and last many days, I can also charge them easely from my single boat battery. By using these Nav lights on an extendable pole above my centre console the light doesn't interfere with my field of view as front fixed Nav lights do.
https://www.railblaza.com/products/ips-light-2/
My emergancy battery is a simple easy to carry lightweight jumper pack that will also power or charge anything else I have onboard
Batteries Direct. This battery pack has been used to start other boats at sea as well as 4x4s with flat battery's at the boat ramp. This style is also used by marine rescue groups over here as they are simply so easy to pass over to people in with battery problems offshore.
I'm about to embark on a trip where I will be away from civalization for a week where I will need to carry water and fuel for the length of my stay (600km trip island hoping the barrier reef 70km offshore). Knowing I have my starter pack incase I have any battery problems either onboard or when back at the ramp with my tow vehicle gives me piece of mind. By the way I've also done away with the dual battery system in my 4x4 as well.