The 3/4 is the thread size and also the rough ball size inside.
The 600 is the pressure rating , usually followed by WOG, for water, oil, gas.
Be carefull as that ball valve is likely brass which will not stand up exposed to seawater. Good bronze ball valves are available from either Groco or Buck Algonquin. Some times known as 85-5-5-5 or red brass but check for the 85-5-5-5.