Even if you touched both positive and negative at the same time standing in water you would not feel much from that battery. The contacs are just to small providing to much resistance making the current flow to low for you to get any real fatal jolt. Your skin resistance (not soaked in water) generally ranges from 2-50 kohms the most current you're going to pull is a few milliamps at most. A 12 volt car battery could give you a bit of a buzz but still not enough to really harm you. In most deaths or electrocutions it's the higher currents that kills not the lower DC voltages but these sure can burn. The perception of electric shock can be different depending on the voltage, duration, current, path taken, frequency, etc. Current entering the hand has a threshold of perception of about 5 to 10 mA (milliampere) for DC and about 1 to 10 mA for AC at 60 Hz. Shock perception declines with increasing frequency, ultimately disappearing at frequencies above 15-20 kHz. In some cases 16 volts might be fatal to a human being when the electricity passes through organs such as heart. Ventricular fibrillation - A low-voltage (110 to 220 V), 50 or 60-Hz AC current traveling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other electrodes), a much lower current of less than 1 mA, (AC or DC) can cause fibrillation. Fibrillations are usually lethal because all the heart muscle cells move independently. Above 200mA, muscle contractions are so strong that the heart muscles cannot move at all.
A much grater concern for your safety with regard to wiring a 12 volt DC power system is a direct short (touching of both bared wires + and - together). Tremendous heat can build up in the wires cuasing serious burns and melting the insulation which can provide fuel for a fire. Effectively getting out of control and burning your house down in a worse case situation. As previously discussed excessive heat and gases from charging have also led to house fires so keep the battery well vented and never leave a charging battery unattended.