With regard to putting the battery in the bilge, before you do that, try one of the following items that might be relevant: sailing at night without lights, starting the motor with without a battery, using your radio when not connected to power, and so forth. My experience is that when a battery is out of service, you are in a real jam, unless it is in the middle of a bright clear day and you are still at the dock. It does not take much water to fill the bilge.

As for AGM batteries, they are great, but when the water rolls over the terminals, they don't work any better than any battery.

I used to have my battery in the lasarette but moved it for two reasons: The most important was that I needed more fuel capacity requiring a second can of gas for the motor. The second reason was the weight of the battery.

My battery location now is mounted high, on a platform extended from the aft side of the aft cabin bulkhead in the port lasarette. The access is through the port lasarette. This is midship (sort of - about the same as under the first step). It is much higher in case of a disaster (and I have an electric high volume pump - so I don't want the battery to be under water). This leaves the area under the steps available for a lot of other things, such as spare gas, a liferaft, food, etc. The battery is out of the way of everything - out or sight, can not be spilled on, can not have a wrench drop across the terminals and so forth.

From that battery location, I have run a #6 cable to the motor along the top back of the lasarette. On the forward side of the bulkhead, just above the counter, I mounted a master power switch. My power distribution panel is located high, under the upper step, on the starboard side.

That's my vote.