Do I choose a series or parallel battery set up to power a fish finder?
Two batteries store the same energy, whether connected series or parallel. If the load can accept either voltage, and deal with it in an energy efficient way using switch mode rather than linear power handling, which it sounds like it can from your assertion that run time is 11.2 hours for either configuration, then there's no difference for run time.

The choice of series or parallel must therefore be made for other considerations.

If the transmission distance was large and wire cross section was an issue, then series will give you lower currents, so lower wiring losses. At your distance with the wires you have available, this is not such a deciding factor. Small vote for series.

Series gives you simpler wiring, your entire circuit can be protected by one fuse and one single pole switch. Parallel needs tee'd connections, ideally each battery is fused separately, and two switch poles to disconnect both batteries. Small vote for series.

Reversion, what happens if one battery dies? As the load can accept both voltages, this can be achieved by connecting just one battery. If the system has to power other loads as well as your flexible input voltage fish-finder, then if they're single voltage capable, it's a strong vote for parallel. If you always have enough batteries to make a full set, then it's no vote.

Cascading failure, can one battery 'take out' the other? That's why we fuse each battery separately with a parallel connection. In a series connection, at end of discharge, the stronger battery could reverse charge the weaker one. Small vote for parallel.

Flexibility, a series connection lets you power either 12 V or 24 V loads. Sounds like a good vote for series. Or does it? Powering a single 12 V load from one of the series batteries unbalances their discharge. For a tiny imbalance it's probably not a problem, but for a significant imbalance, it limits the lifetime you can achieve on the main load, and promotes the reverse charge at end of discharge problem. If you can be disciplined and refrain from using the 12 V centre tap for significant loads, then it's a small vote for series. If you're tempted to use a 12 V light, then it's a bigger vote for parallel to prevent you from unbalancing the batteries.

Different considerations will have different weightings for different people. As you see, there's no strong reason to choose either. If you find one of those reasons suddenly becomes stronger, say you want to power your fish-finder in a remote drone boat, and need the higher voltage to use thinner wires, or you want to run your 12 V TV as well so need parallel, you might as well just flip a coin. You may have to keep flipping the coin until it comes up the way you really want!