Here is your marine tinned wire from same place. You can have this one premodern with battery ends installed. Couple dollars more but worth it in long run.
6 Gauge AWG Battery Cable Wire - Solar Marine Power Inverter Car Pure Copper
Looks like this stuff is the same as marine wire. I think this is a good price.
Welding Cable Red Black 6 AWG GAUGE COPPER WIRE BATTERY SOLAR LEADS
Here is your marine tinned wire from same place. You can have this one premodern with battery ends installed. Couple dollars more but worth it in long run.
6 Gauge AWG Battery Cable Wire - Solar Marine Power Inverter Car Pure Copper
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Charlie Weaver USN/ENC 1965-1979
Marine Grade Wire Duplex DC, Tinned Copper USA Made Boat Wiring
For use in a marine application tinned wire is Best. It is not as likely to corrode in a wet environment. This website has all types of marine wire and connectors.