I've been using one of these
Bass Pro Shops XPS i Series Advantage On-Board Marine Battery Chargers for several years with no problems. I have 3 batteries so I use the XPS 5/5/5. Some people will say you don't need a lead for the starting battery but I prefer to have one. Most outboards have very low charging circuits and many times the area I fish does not require running the big motor long enough to keep it fully charged. So by having the on-board charger connected, I am assured of always having a fully charged battery. And yes, you can just plug them in and forget them until the next time to go. I've heard a lot of good things about the Optima batteries and I use one for starting. But never tried them for trolling as I prefer the Delco Voyager batteries. Whatever one you get, get a deep cycle with the largest reserve amps you can. This will ensure you of max trolling time.