Is there a quick fix for this besides installing a second batery?
Do I need to install a diode in the accessory circuit to prevent the
started from sucking power back from the locator and stereo?

Clean the terminals to the battery. Clean the terminals on the starter.
Replace the wires to the starter (both positive AND negative). This is
about the extent of 'quick fixing'.

Possibly use heavier gauge wire to the "locator" (chartplotter or
something?). This to make sure it's getting enough juice anyway. If it's
using too small a gauge wire it may not be getting the right level of
voltage to begin with.

But if you've got these devices on the same battery you really should be
TURNING THEM OFF before cranking the starter.

Your best solution is to get a second battery installed. It may be simplest
to get a new one just for cranking the engine. That way you'd probably be
able to leave everything else wired to the existing battery and only have to
rewire for the starter/ignition.

Having a second battery helps make sure you don't end up stranded somewhere
after spending all afternoon/night on the hook listening to the radio...