If your electronics go off when you start your engine, you have a wiring size and/or connection problem that should be corrected.
Deep cycle or combination batteries for battery bank
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The only time this should be an issue is when:
1- You are practicing poor battery management and let the bank get below 50% SOC
2- You have batteries in poor health.
3- You are trying to start and do house loads off a very small battery like a single G-24.
4- You have wiring in poor health...
Where this benefits boats the most is with small sport fishing boats who start the motor multiple times per day and power electronics all off a single small battery. The same can be said for small sail boats with small single house batteries.
With most cruising boats that have adequately sized house banks, consisting of two or more batteries, you will almost always see less voltage sag on a house bank at 50% SOC than on a single battery at 100% SOC... I have done the comparisons, with the equipment to measure it, many, many times.. As the house bank gets even larger, 250Ah +, there is no comparison between a single battery and the house bank even at 50% SOC. The large house bank barely even notices the motor start.
There is nothing wrong with isolating the start battery but most boats are not wired for it and the added cost is often not necessary.
In 20+ years of always starting off the house/everything bank I have never once dropped out a single instrument and this includes starting a large turbo diesel Cummins on 6V T105's in mid winter........
I've also yet to measure the elusive a damaging "spike" across a properly wired 12V battery bank even with an o-scope. Always looking but yet to measure one...
If you are dropping stuff out, and have an adequate house bank:
1- Address the issues in your system
2- Isolate the start battery and ignore looking for the underlying problem...
If you are dropping stuff out and have only a single battery house bank:
1- Isolate the start battery from the house..