As a check for the bulbs.
Are they the same as the stern light?
Does the stern light work?
If yes, try the side light bulbs in the stern light fitting, that should rule
out the bulbs.
As you said you are an amateur when it comes to electrics
, when you check the contacts in the side light bulb holder, make sure you have the multimeter set to measure volts DC, it probably has a setting for 20V DC which, so long as your boat is 12V, would be OK.
Check both the condition of the bulb holder contacts, and the ends of the wires going into the bulb holder. If the previous owner used non-tinned cable, it is likely that the cable ends will be corroded which would cause the bulbs to not light up.