That is the engine breaker for the 12VDC system, 50 Amp. Use a voltmeter and see if there is 12V at the breaker (both terminals). No voltage check 90 Amp fuse on the starter. Voltage there check for 12V on the ignition switch B terminal. Then 12V on the S terminal with the key in the start position. Finally 12V on the yellow/red wire of the slave solenoid which is near the engine breaker. Report finding and we can go from there. Also under Documents of this site you will find the Mercruiser manual for your engine, big help.
Worse lines to hear when buying a boat; It just needs... battery , carb cleaned, ... Typically much worse.
1997 2400 SCR
5.7 Vortec / Bravo 2