Go to this website.
And you can read the whole thing, but read down to where it says.
the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages).
The battery is a 6 cell battery (for 12v) and float charge at 2.25V per cell is 13.5, (2.30V per cell works out to be 13.8)
But note where it says summer temps may require lower voltages as well. Voltage creates heat, heat creates gas. The batteries are sealed, but charge them up high enough and they simply melt, and then explode.
When charging an AGM battery, there are 3 phases or stages to charging an AGM battery, bulk charge, absorption charge, and float charge. This is a lot to get into, and you need an electrical engineering degree to figure it out. (even I don't know it all) But think of bulk charge as charging an initial dead battery, with any dead battery you can throw 15V at it without a problem until it starts to take a charge. (this is likely what you read on the internet) After that you have to turn the voltage down and there is "Absorption charge" which is charging a battery that is 1/2 dead and getting it closer to full charge, this is the middle ground.
And then float charge is maintaining the batteries voltage once its up above 85% or so.
Now when it comes to outboards, outboards do not have a 3 stage charging system. (because lead acid batteries do not require it). They charge a battery just like a cars alternator does, in bulk. There is no way to regulate and taper off the voltage for AGM batteries. So if you took a dead AGM battery, hooked it up, an somehow rope started the engine you would be okay for an hour or so. But if you took a brand new fully charged AGM battery, hooked it up, starter the engine and then took off for an hour, for that hour the battery is going to be sitting there overcharging. (and getting very very hot). And eventually it will pop.
Another way to look at it is if you go to the tool store shopping for just a battery charger, AGM batteries take a special charger, and will not work with a standard battery charger. The 50 and 60 dollar chargers will not work with AGM batteries, it will cook them.
AGM charging is very sensitive, and takes specialized chargers. The engines charger was never designed for an AGM battery.
Does that help make better sense? If not, just ask, as i'm here to help.