I think there has to be something else going on here. 12 volts does not penetrate the skin, even if it is wet. It can send current over the surface of wet skin, but it does not penetrate.
A deep cycle battery has less available current than a normal car battery. It is designed to provide less current, but for a much longer period of time. Something else is going on here. If the OP was on a dock and reached into a boat that had been traveling through water, then he(she) could have gotten a bad static shock, but thats due to a build up of static electricity from plowing through the water, not from the battery itself.
One more little detail, the battery is DC. DC is inherently safer than AC. A little demonstration that the Edison group used to do back during the “Current Wars” was to shock a dog with 500 VDC. It would startle it for a few seconds, sometimes cause a small burn, but it didn’t do any lasting damage. The they would hit the dog with 110 VAC and the dog would die. This was done to scare people away from AC and go to DC. In fact the electric chair was invented by the Edison group to use AC to execute convicted criminals to show the public how dangerous AC was to have in their homes.
The frequency of AC electricity, 60 hertz, is very close to the frequency of the nerve impulses to control our heart. That is why it so often disrupts the nerve impulses, leading to fibrillation, which is an irregular heartbeat that does not push blood through our bodies.
DC burns. I was working on a power supply once when I accidently sent 500 VDC through my body. I never felt a shock, but when my hand started to burn where it was grounded, I felt that.