What is the KAC-531 best used for?

It is a mono bass amplifier, so it drives one subwoofer. It is not intended for door speakers or full-range audio.

Does it fit a stock electrical system?

Class D amps are more efficient than Class AB, so current draw is lower for the same bass output. That helps on cars with a factory alternator and battery.

Can I adjust the crossover?

The low-pass filter works in the 50–200 Hz range. Set it near the upper end of your subwoofer's rated range so the sub blends with the door speakers.

Is it suitable for a boat?

It is sold in the boat and car amplifier category, but check that the mounting spot stays dry and ventilated. Marine use needs protection from spray and moisture.

What subwoofer should I pair with it?

Match the sub's impedance and power handling to the amp's rated output at that impedance. A single sub in a sealed or ported box is the typical pairing.