We looked at Robters and their site stated - Please note that this tour departs from Hilo Airport and Hilo hotels only.
I looked them up on some chat forums and from what i have read, Roberts is tied to one of the cruise ships and cant tour other cruisers. They are exclusive with one of the cruise ships.
Anyhow, our tour with www.hawaiicruiseexcursions.com starts right from the cruise ship port and is $70 each. We were asked for all of our names of everyone in our party as our tour guide will be inside the port, waiting at the ship for us. my kind of customer service!