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OMG - a series ammeter?
They junked those decades ago in cars for 2 main reasons.

One was the hazard (and voltage drop) associated with running the a heavy cable from the battery +12V to the dash ammeter and back to the alternator and fusebox.
That has since been overcome with remote sensing which measures the voltage drop across a shunt resistor else a known cable. (Voltage across it is proportional to the current thru it, ie V = IR.)

But the second reason(s) was the clincher.
An analog ammeter couldn't show small battery currents, and most of all, they didn't tell much about the charging system and battery condition.

Enter the voltmeter.
That uses 2 thin wires, +ve & -ve (or GND) for ~1uA, or ~10mA if digital.
And it shows if the charging is ok, eg, 13.8 to 14.4V under normal circumstances (or down to ~13.2V if "smart regulators" are used with fully charged batteries).
It also shows the battery condition, ie, ~12.7V down to ~11.5V. That's with no load, but you soon learn the normal battery voltage with a load. (EG, 12.7V may drop to 12.4V with IGN or loads switched on.)
Over-discharged will be down more than ~0.2V (20%) for a normal cranking battery, or down ~0.5V (50%) for a deep cycle.
Over 14.4V long term is boiling or wrecking the battery.
Larger cranking dips over time to 10V, 9V, 8V etc indicate an aging battery.

I would have expected voltmeters to be even more popular in boats (than cars etc) due to their "stator" type charging systems (ie, brushless; no voltage-regulator controlled rotor regulation of output, hence full-current charge-pass or charge-dump regulation) which can be more prone to voltage issues (adjustment creep etc).

Note that the voltmeter should be wired across the battery terminals since that is what the charging system & voltage is based on. That means a permanent drain (maybe as much as 1uA for an mechanical voltmeter) unless switched.
For my 7mA digital voltmeter, I use a (tiny) relay which is energised by IGN +12V (or a manual switch or timing circuits).
Voltmeters should not used the vehicle chassis/body/bash GND or IGN/gauge +12V since they are likely to vary and under-indicate the battery voltage.

Of course it is here that I ask if you have a voltmeter so I don't have to write the above for those in the know.

But since you have an ammeter, I assume you don't have a voltmeter since then there is no use for an ammeter (except for occasional "experimental" purposes)...

The volt meter means you have more info about your system.
The "useless" ammeter can then be bypassed or junked; thereby less wiring, less hazard, and less voltage drops to and from the battery.

Incidentally, "equal length" - or rather, equal resistance - is only important when balancing is involved, - eg, permanently paralleled batteries etc.

In vehicle systems where the chassis/body is often used as a GND or -ve battery return, the -ve/GND cabling often exceeds the +ve feed capability.
This is partly due to other currents (alternator GND thru engine & chassis etc), but also to prevent damage to electrical/electronic equipment, melting of throttle cables and any other ground paths & wires due to a bad GND.
Often multiple engine-chassis GND cable are used so the loss of one does not cause damage.
But that is not an issue in non-metalised "grounded" boats, though loss of a GND could effect or damage interconnected electrical equipment (radio to a separate amplifier, etc).

But a similar rule applies - all cabling with as low a resistance as possible (hence shorter, else bigger). And "excess" grounding can only help (except in ground-loop cases, but that's a path issue, else GND cabling being too thin).

Apologies for the lesson-101, but once understood, it makes ammeters VERY unattractive.
Though I do miss my dash ammeter - it was a bit of oldskool bling. I have thought of hacking it to indicate battery charge & discharge rate, maybe as a remote sensing ammeter (from the -ve or +ve battery terminal) though probably based on actual battery voltage.
Maybe I'll use it as an accelerometer instead?

battery cable