2 things about that amp that would have me keep shopping. Specs are sketchy, but based on the size of the single fuse, im guessing that amps RMS will not be much more then that of your head-unit. So no real benefit. 2) it looks to be full-range only, no selectable cross-over filter.
To go one step further, im not sure it is bridgeable, so no chance to drive a sub with it.
So to answer your ?'s
1) neither really, but will not be any better then the head-unit
2) not with this sub, you will need another
3) Clarion deck is a solid H/U. This in-boats are fine if they fit the boat, budget, like the looks and sound. Amp, well i think we covered that.