Hello I bought a smart 2/4/6 amp smart charger to charge my marine and motorcycle battery. When I plug it in to charge it get up to 15.7 V is this going to wreck my batteries? I always thought charging above 15V was bad but I can only adjust the amperage. The voltage requirement is determined by the charging unit. Am I ok to charge 12v batteries above 15 V?
Yes, and you should find that the charger will adjust the voltage and current as it needs to as it senses the battery state of charge.
My experience was that cars should charge 13.8 to 14.4 volts (14.6 for dynamo) but this has changed with the newer generations of batteries and charging systems. My car (2005) will charge - especially in winter - during the first 5 or so minutes at 15.2 volts and a fraction more - but I don't get time to watch ... have to watch the road..
You should not try to measure the "unloaded" output of a basic car battery charger. (It sounds like that's what you've done). Hook it to a well charged battery and measure that voltage. (Or monitor the voltage as you charge a depleted battery.)
If it is actually charging up over 15v you are correct that would be bad for the battery.
I'll add car chargers with a "booster" range ( 50, 100 amp claimed ) quite likely do overcharge if you were to leave it on that setting.