Deep cycle batteries are better suited for discharging and charging lots of times.
Marine "cranking" batteries are "suppsed" to contain heavier plates and more support for the plates to keep you from shaking your battery apart.
What I have found, though, is that a boat that doesn't sit and drain the batteries deeply (mega stereo, etc) doesn't need a deep cycle battery at all. I've also found that I can shake a marine battery apart just as easily as an automotive battery.
I've become a believer in battery WARRANTYs. Whoever offers the longest Free Replacement warranty, followed by the best prorated warranty gets my battery business.
Back when I DID buy marine batteries, DEKA gave me the best life. Die Hard gave the worst. None were anything to write home about.
I'm on my AutoZone Gold 3-yr Free Replacement batteries in all my boats right now. April 2000 for all seven on the Sea Ray diesel, still going strong. July 2000 for the two in the Formula - still going strong.
I've also gone to replacing with them in all my cars and work trucks. The oldest are the ones in my F-450 Ford/International Diesel that were installed in 1998.
I honestly see no reason for me to ever buy anything else.
Now, with sportbikes, you gotta use a sealed battery. The factory sealed lead acid batterys are only "fair". Seldom-used bikes will eat the batteries prematurely. Sulfation gets to 'em. I do plan to try an AGM battery the next time my R1 needs a new cell, although the AGM didn't perform up to my expectations on the jetskis.
In 2000, I decided to stop playing the "batteries on the shelf with a trickle charger all winter long" game with the jetskis. I replaced the batteries with expensive AGM batteries because they were supposed to be able to withstand deep discharging a zillion times and be able to be frozen solid and brought back from the dead. That's a lie. They keel over just like a cheap-o. These days, I've gone back to cheapies and trickle chargers...
Here's my take on Gels.
Gel batteries cannot be "fast-charged" cause the electrolyte WILL boil if you do so and it will vent out of the pressure valve and then the battery will be on a short fuse. Gel batteries also require the charging system (alternator, regulator, external charger) to be set to a lower float voltage than a flooded battery or guess what (the electrolyte will boil out).
Once it boils out, you got expensive unwarranteed paperweights (warranty does not cover batteries that have been overcharged).
I had an outboard that I was rough on. I tore the transom out a couple of times and busted the stringers out on another occasion. I was having a devil of a time keeping the batteries from being shaken to bits (happened lots of times).I finally cut the bottom out of a cheap styrofoam cooler and made a cushion to sit the battery on. I clamped it into the holder on top of the foam and never shook another battery apart again.