Solar Trickle Charger

The 21 volts is apparently the open circuit voltage for your panel. It is all voltage, no current. With a load on it (the battery) it dropped to the lower voltage.

The charge controller should be using a high enough voltage to deliver the charge to the battery. But at the same time, it uses a lower voltage to provide maximum watts to the battery. If it just cranked away at 14.8 volts, it would deliver less watts than it will at lower voltages.

Your trickle charger is not going to charge a marine deep cell battery that is at a significant discharged state, which I suspect from the 11.8 volt reading. It's real intent is to combat the self discharge of the battery.

I would but the battery on a charger and bring it to full charge. Then connect the solar charger to maintain that state of charge. I believe you will see a higher charging voltage with the battery at a higher state of charge.

25 watts at 12 volts is just over 2 amps.
That is also dependent on having full sun. With the sun lower in the sky, it will produce less. Air pollution or fog, clouds etc, can affect the output.

At this time of year in the northern areas, you might only get the equivelent of 2 hours of full sun a day. In other words, about 4 amp hours.
Southern areas might get 4 hours of full sun, or about 8 amp hours. These figures will go up in the spring/summer.

Those amp hours are based on the panels potential output. What the controller puts into the battery may be lower.