I'm thinking of adding a second battery to my 18 foot bowrider. I want to
use one battery for the engine and one for the stereo and lights and air
pump and ...

I don't want to lose my engine starting ability due to a battery that's been
run down using accessories.

My first battery is a Marine/RV Cranking type (I can't read the labeling to
know what it's amp hour rating is).

What type should my second battery be?

Thanks