1

If seas are calm it will be fine
If seas get rough it can be hell

Why not just take the larger, safer ferry?

2

Like above said.
But I think go by Ferry is better way for comfortable seat.

3

Resist the temptation to sit out front on a speed boat. Looks like the ideal spot until the front starts slamming down loosening your teeth and brain. Much better toward the back but don't sit in the very back next to the engine which will get noisy and possibly exhaust. The back corner can also be bad because as people crowd in they crush you with no leg room. Sit on the side and when people get in make them pass you for the back seat instead of letting them slide you into the corner.
A day trip boat to the island is better than simply a speed boat "bus" because the the day trip has a limited number of people. The "bus" type just cram people on until it is seriously overloaded. They don't seem to have any idea of what full means, and making it worse is the crazy amount of luggage the tourists haul around.

I agree with RiniRabit that the ferry nice. Strangely it costs less to buy the ticket including taxi to the pier in Krabi town. You can get the ticket at a tour shop, hotel or guest house. I stayed several places in Krabi and liked the Krabi River Hotel.
I like Krabi area, lots of good info here: http://www.travelfish.org/location/thailand/southern_thailand/krabi/krabi
favorite day trip from the area is Phang Nga, so beautiful.