Both my engines have the NOS Mercury starters (496 Mag HO) they were replace 2 years ago and have 85 hours on them. My batteries are Kinetik 1000 AMP batteries and ran low once or twice from leaving the stereo on or whatnot. They seem to really struggle to turn my engines over EVEN after I charge the but especially if they are warm. I assume this is a ground issue? Since I really cant get to the starters with the engines in the boat what grounds do I check first. Could the batteries just be loosing their punch? Or is it more likely a ground issue? Both engines start just as hard so I am pointing to the batteries... Where do I start to diagnose this?

If you havent done so already, I would load test the batteries

Disconnect both batteries and let them sit over night. Then check the static voltage. You have to let them sit because if you charge them you'll get a false reading. The batteries should read about 12.5 volts. If not pull them and take em to Advance Auto. They can do a load test for free and tell you what's up. If the tests come back good, then check the grounds on the back of the engine. Make sure all the connections are shiny metal free of any corrosion. I like to smear dielectric grease on the connections then reinstall them.

Hope this helps.

Don't load test an AGM, go somewhere that can do a internal resistance test. Sounds like you are having the same problem that I had years ago with AGM, the main reason why I switched back to lead acid. The AGMs are lighter, can be installed anywhere, and vibration doesn't bother them. However, for abuse of overcharging or overloading and draining down to nothing the lead acid will always win. When you abuse a lead acid you boil out some water, when you abuse an AGM gas escapes that can not be replaced.

I found AGMs are not a good option for a boat that is idle running tunes and lights, lead wins. 1000 amps is 1000amps no matter what.

I run large (not sure what group) Napa deep cycle lead acid about 1000 amp, not the dual purpose, the full deep cycle. They last about 2 years and napa replaces them free when they fail. So I have $250 invested in batteries over the last 6 years, I'm on my 3rd pair.

Yeah, they ran dead once or twice. When they recharged then "bubble" and I assumed this was bad. Unfortunately, I strangely suspected they were turning to junk.

I am still going to check the grounds, but its almost easier to take the chance and put new batteries in... For some reason, I just never feel like crawling around the engine compartment anymore.

check for corrosion at batt terminals.

Also I stick with regular Marine Die hards- had best luck with them. AGM are not worth the extra $ IMHO.