What amplifier power does the Focal P20FSE need?

It is a passive 4-ohm subwoofer, so match it to an amplifier rated for that load. Focal publishes a recommended power range for the Flax Evo line; stay within it to avoid overdriving the driver.

Does it work in a sealed or ported box?

Focal lists enclosure recommendations for this driver, and both sealed and small ported boxes can work. A sealed box gives tighter, more controlled bass; a ported box trades some accuracy for more output.

Is the Focal P20FSE a good upgrade over a factory 8-inch sub?

Usually yes if your car has an 8-inch factory location and you add a proper amplifier and enclosure. The flax cone and Focal motor give cleaner, more controlled bass than most stock drivers.

Can I use it in a boat?

No. Focal does not rate this subwoofer for marine use, so moisture and salt air can damage it. Choose a marine-rated driver for boats.

The subwoofer itself, with mounting hardware and documentation. An enclosure, amplifier and wiring are not included.

How does it compare with a 10- or 12-inch sub?

It gives up some deep, loud output to larger drivers but fits where they cannot. Choose it for space and accuracy; choose a larger sub if maximum bass is the goal.