The "Marine-ness" of a battery isn't necessarily the same as its "deep-cycle-ness." "Marine" may also mean that the battery has SS threaded terminals for instance. Also, deep-cycle is not a true/false sort of thing. There is a spectrum of "deep-cycle-ness."

BTW, just having an Off-1-2-Both switch doesn't provide any real isolation, since you have to put the switch in Both to charge simultaneously, or keep fiddling with the switch. To get the full redundancy benefit of 2 batteries, you would need a dual-output alternator and an isolater. $600 later ...

You may want to consider a single large spiral-cell battery like the Optima D31M -- lightweight, compact, plenty of cranking amps, plenty of capacity, no switch to fiddle with. Yes, if you leave the lights and electronics on for a long time without the engine running, you will drain the battery and be SOL.