Is 6.0 peak HP enough for a garage?

Yes for typical garage work: sawdust, dirt, leaves and small wet spills. Peak horsepower is a peak rating, not sustained power, so judge it by tank size and airflow needs rather than the number alone.

Can it pick up water?

Yes. It is a wet/dry vacuum, so it takes liquid pickup as well as dry debris. You switch the setup for wet use, and the 8-gallon tank gives you decent capacity before emptying.

How does it compare with a smaller 4 to 5 gallon Shop-Vac?

It holds more debris and has a stronger motor, but it is larger and heavier to move around. A smaller unit suits quick car and corner cleanup; this one suits bigger, dirtier jobs.

What hose size does it use?

It uses the standard 2.5-inch shop vacuum hose size, so common 2.5-inch wands and nozzles fit. Confirm the exact fittings in the manual before buying accessories.

Does it come with attachments?

The listing does not spell out the included accessories, so check the box contents before buying. Most Shop-Vac tanks of this size ship with a hose and basic nozzles.