Battery & Battery Charger Help..

Hey guys, I'm new to the boating world just got my first boat on sat. The engine start battery on the boat was shot and from back in 2005. And the deep cycle marine was also getting bad and from 2008. I went ahead and got two new batteries from Autozone and a charger. They guy told me to go with a 50amp or 75amp charger so this is what I got.http://www.autozone.com/autozone/accessories/Schumacher-75-20-10-2-Amp-speed-charge-automatic-battery-charger-engine-starter/_/N-2604?itemIdentifier=58755_0_0_
Just want to know if this is the right charger. It has options to for all the battery types. So when I put the small engine start on to charge I put it on that setting. Then when I put the deep cycle marine I put it on the AGM and large setting. Any help would be great. Thanks

 

I have a deep cycle 24DC marine and a small engine start battery. I know the charger has all the different setting for every battery, I change them when switching batteries. I just thought when he said I need a 50-75amp why not get the 75amp since it was only a few $ more