I'm having an issue with batteries. Twin 500 efi's and brand new batteries that are fully charged. Battery voltage is 12.7 volts. I put a volt meter directly on each battery and when the engine cranks the voltage drops to between 9 and 10 volts. Stop cranking and the voltage is right back to 12.7 volts. The same thing is happening on both motors. The batteries are Duracell size 29 deep cycle with 810 cca @ 32*. Anyone have any ideas on where to start looking? Thanks.

Not a battery expert, but do have some knowledge, if your battery is a true "deep cycle" battery, then you bought the wrong battery. Deep cycle batteries are for trolling motors and house loads.

I would suggest you need at the least group 31 (staying with Duracell and Sam's club AGM) , or better yet get Lifeline or Trojan brand

I've run the same size and type batteries for the past 3 sesons with no issues until the end of last season. Had one battery go bad my last day out last season so decided to replace them both.

I'm just looking at the Duracell site and it shows the 29 as a deep cycle, and the 27 and 31 as starters. That said, start looking at your ground cables, but pulling the bat down to 9V is in most cases the battery or starter. Try cranking and if it cranks for a few seconds, quickly touch the cables and see if they are hot. If their getting hot then it may be the cable connections

There are also 2 battery switches wired in - could one of the switches be bad and causing an issue with both batteries?

Doubt it, because unless your connecting them together there is no path for current flow. To follow on the statement, I don't know how your wiring is setup, but since this is happening to both engines, and it didn't happen before, it has to be wiring or batteries

How well does the boat start as it sits now with the new batteries ? Or does it not at all ?

Its starts very hard. A long hesitation when you turn the key and then it cranks slowly. Seems to start much mote normal when battery switch is set to "both."

Ya know you might just need new starters, but still think this is a wiring or bat thing.

I just changed out this year from original Merc to permanent magnet starter and boy does it spin over fast now

Good point. Seems odd both sides but had stranger things happen. Not sure what kinda switch you have but does it make a difference on all position or try jumper cables from battery to batttery. If your not noticing much of a diff I would have to agree with potential starter issues. Boat starters see a lot of moisture and condensation. I recently bought a battery load tester from NAPA. Can't believe I went all these years without one. As mentioned double check engine grounds from battery which I'm sure you've done already.