I have had these kind of problems in the past and it has always been a voltage drop in the wiring because of the heavy starting load.
Go over the wiring closer looking for bad connections especially at the battery and any inline switches or bus bars.
It could even be a sign of a failing starter motor or dying batteries.
The best situation is to have an independent house battery that powers all your electronics. This kind of a spike has the potential to damage your electronics. I remember once I did a setup situation with diode and a capacitor. The diode in series with +12volts feeding the electronics and a capacitor on the electronics side of the diode. The capacitor needs to be big like 10,000 ufd rated at 20 volts minimum. This configuration serves to provide a little voltage reserve or buffer to the electronics so that when the starter drags the voltage down on the battery side of the diode the capacitor holds the voltage up for a second until it returns to normal. Of course if your cranking for a long time you would need a proportionally larger capacitor. As I said above best thing to do is to set up a house battery that runs your electronics and is not loaded up with the starter, you will need a diode splitter from the alternator to charge it. Depending on how much electronics you run off it it can be a much smaller battery than your cranking battery but should be deep cycle so you c an run it down with out damaging it. Good luck , happy boating