Marine Starting batteries and Automotive batteries are basically the same, except for the additional screws that marine batteries usually have which allow for quick connection/disconnection of equipment.
Like the above user said, dual-purpose batteries usually are either better as SLI batteries (Starting - Lightning - Ignition) or as Deep Cycle batteries. However, there is an exepcion to the norm: Optima Batteries.
Optima manufactures AGM batteries that are really excellent Deep Cycle batteries while also being even better Starting batteries than the conventional ones.
Among its many advantages, Optima (BlueTop, YellowTop and RedTop) batteries last longer, MUCH, much longer that conventional ones (I've seen Optimas 5, 7, 10 and even 13 year old still working like new even in very harsh environments); they re-charge 300% faster; they retain their charge for months and months (up to a year) when not in use (regular batteries are dead before 2 months of non-usage); they provide much more power (Amps) and energy (amp-hour) than regular ones in about half the space and with less weight; AND given the fact that they are hermetically sealed, don't gasify, do not pose a fire risk, are not affected by humidity (can work even under water) and do not pollute.
I've personally overseen the installation of hundreds of marine batteries in vessels of all sizes down here in Puerto Vallarta, and can attest to the reliability, performance and longevity of Optima. If I were you, for your boat starting battery I would choose an Optima RedTop (it costs less than $100 here in Puerto Vallarta, with free delivery and installation in your boat).
Feel free to e-mail me for more specifics or tech details, my website is
http://SoloBaterias.comm.mx/
