Is the Kicker 44VL7S122 a complete sub and amp package?

Yes. The 12-inch square sub comes mounted in a vented MDF box with a built-in amplifier, so you add power, ground and signal wiring rather than a separate amp.

Can I use it in a boat?

Kicker does not rate this model for marine use. Its MDF box and non-marine components are meant for car interiors, so a wet environment will shorten its life.

How much power does it need?

It is rated at 750 W RMS and 1500 W peak. Plan a power feed and wiring sized for that RMS figure, since the built-in amp draws real current at high output.

Will it fit in a small trunk?

Measure first. The vented MDF enclosure is bulky, and the port needs clearance, so compact trunks and hatchbacks may need the box shifted or the cargo area rethought.

How does it compare with a round 12-inch sub?

The square L7 cone has more surface area than a round 12, which typically means more output for the same footprint. A round sub in a custom box can be tuned more freely.