Your battery should be in the range of 12.8 to 13.2 volts if it's fully charged. The voltage should be checked when the battery has not been used for a number of hours, overnight would be good. Take your digital volt meter and check, let us know what you have. Also know what the temperature is when you take this reading. Report back with that info.
Do you have shore power available? Is the boat in the water? Do you have a battery charger? Some chargers have a start position so if yours has that, you could try starting the engine with the charger in that mode.
If your battery voltage is much below the above values, you need to charge your battery. It may also be in bad shape. See above comments by others.
If your battery has correct voltage, then wire connections are your next thing to walk through, also as said by others above.
Let us know what you find.
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