Corroded/resistence wiring someplace. Deep cycle vs start type battery makes absolutely no difference.

Thanks. I agree - I've always used deep cycle batteries for starting and never had an issue.

A voltage drop to 10 Volts at the battery while starting is completely normal, especially on a starter that's spinning slowly. The slower the starter is turning the more current it pulls and the higher the voltage drop across the battery's internal resistance. If you feel the starter is spinning abnormally slow check the voltage at the starter terminals while turning. Any excessive voltage drop in your wiring will show up there, not at the battery.

Those EFIs don't need some specialty 1000 cca starting battery and your deep cycles should do fine.

The probability of two separate engine's wiring systems going bad at the same time is small. Stating there are no differences in the batteries is just plain
wrong.

Some guys had a happy outcome when putting their boners through a hole in a porta-potti. Others got a rat trap!
It doesn't mean you should take the chance.

Your saving grace has been your on board charger. A fully charged deep cycle battery will start an engine but it is not ideal. At what point does a
standard deep cycle battery not start a high compression/high horsepower engine? I don't know. Ask Mercury or one of the pro engine builders.
Now that your on board charger is not performing you are experiencing what most of us that have tried these things have found out. Your engines
alternator will not fully charge a deep cycle or dual purpose battery when just tooling around in the boat going to the beach, bar, or your GF's dock.
These batteries need many hours on the charger to get a proper charge.

Dual purpose batteries would be a better alternative for you.
The best scenario is what I described before.

Replace your on board charger and use it every day and you will probably be O.K.
Do things the way I first described and you will be O.K.

I have used all three types of batteries on my boats and found that using 1 starting battery and 1 deep cycle is the best way.(single engine)
I also remove my batteries and take them home to be put on the charger on a regular schedule.

If I'm not mistaken these are the specs for the OP's engine;
HP500EFI Specifications

Propshaft Horsepower: 470/350
Displacement (Liter/CID): 8.2/502
Cylinders: V-8
Bore (in/mm): 4.47/113
Stroke (in/mm): 4.00/102
Compression Ratio: 8.75:1
Fuel Requirements: 87 Posted Octane (R+M)/2
Full Throttle RPM Range: 4800-5200
Ignition Type: Digital
Alternator (amps/watts): 60/847

Not exactly 'high compression'. That shouldn't require anything more than an average 600 CCA battery. Although I agree, the dual purpose AGM batteries made today are the best choice for almost everyone.

Are these 500s still turning belt driven air cleaners? When I had M3s on my 500s if the battery was in any way down or bad connection it did the same. To be honest, I bought a spool of OO gauge wire this winter and built new cables. My BT was originally setup with a shared ground between the engines then back to 1 battery. Now both motors go to an individual battery. It made a difference.

I think Chris means hair dryer's lol

I wouldn't like that shared ground but very well could common but an easy fix also.

WHAT ?? never mind