Never tried SeaFoam but I can vouch for B12... Be sure to go by the mixing instructions on how much B12 -to- fuel
I believe Yuasa agrees with you...Most fully charged batteries measure around 12.75 volts. Some a bit less, and now and then a bit more.
They will measure higher when just disconnected from the battery tender. Let sit for a while and check the volts again.
Half flat or half discharged is 12.25 volts. If you consistently get 12.25 or less, the battery is tired.
I agree with that although as low as 13.5 and even up to 15.3 could be common for some machines when running above about 1500 RPM, or higher.With the engine running, 14.5 is common. Bon chance.