A second battery is certainly nice, but it's probably not a necessity.
A portable jump pack can be had for $60 and has utility outside of just the boat. Most of them have cigarette-lighter outlets as well, increasing the utility.
You'd have to be spending all day at a sandbar to risk running your battery low (IMO).
This is one of those things that might be worthwhile if you can get it done for $400 or less. Beyond than, I'd probably reconsider it (you can also always add it later).
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