I recently bought a 2008 g37 coupe was diagnosed by a shop to need a starter, when I got it it would crank one time then stop. I replaced the starter same symptoms, bench tested the new starter and it works fine, tested voltage when cranking drops from 12v to 8v. Already tried the battery out of my running car and produces the same symptoms

Started chasing grounds to clean them up and found water in both passenger and driver floorboards. Checked ECM BCM and IPDM all seem to be fine no signs of water intrusion

Replaced negative cable terminal to ensure tight connection

tested all fuses

ran the negative cable of some jumpers from the negative to the engine block and same symptoms

engine turns over by hand and I pulled the plugs to make sure it wasn’t hydrolocked

running out of ideas of what to try maybe someone has had this issue in the past?