Hooked up batteries incorrectly... How much damage did I do?

I have a dual batter setup in my supra with them connected in parallel to a perko dual battery switch and had to replace a battery. I accidentally hooked up the two batteries in series and doubled the voltage to 24. I even started and ran the boat for a couple hours before I caught my mistake. I rewired it and it is correct now, but I am not sure if I messed anything up or not by running at double the voltage. There was a bit of a whine coming from the engine bay and the engine temp ran a bit high but other than that, I did not notice anything when running at 24 volts. Can anyone think of anything that would have been ruined from my mistake?