[Mee-n-Mac]

The best answer depends on how you use your boat. Dual batts are nice because you can have 1 deep cycle and 1 starting batt (or a dual purpose) and use each when it's needed, having the other as back up. Wiring a 2'nd one in is a piece of cake. If you don't spend a lot of time on the lake with the engine off and juice on, or if you don't have the space for a 2'nd batt then the added complexity of having to remember to manually switch them won't be worth the hassle. There are more complex schemes that take the memory out of it but they can be $$s. My guess is this (dual batt) route is overkill for you.

A battery saver is a good thing, also easy to install, and doesn't need as much space as dual batts. Good thing to have if you drift about on the lake with the radio on and engine off. If you go this way and want a hand installing it, let me know.

The jump pack is more versatile as it can be used on the boat (if you remember to store it there) as well as off the boat. Might be handy on the island to have a portable, spare 12V outlet. I'd say go for the jump pack.