Jennifer Bakowski told Phuketwan she aimed to ask the company more questions in Pattaya directly in seeking answers about her brother's disappearance on April 11.
''We're pretty disappointed with the way that the dive company and the boat handled everything,'' she said. ''They left the scene and continued on knowing they had left somebody in the water. They've got some explaining to do.''
Jennifer Bakowski and her mother, Marie Gallant Major, flew in from the US to Phuket on Sunday after supporters paid for their trip to seek answers about the disappearance of 36-year-old Joshua.
After talking exclusively to Phuketwan, she and Mrs Major were riding a search helicopter with Phuket Governor Nisit Jansomwong and other officials today to see for themselves the area of sea in which Joshua went missing.
Both still hope that he could have made his way to shore or have been picked up by a boat.
''We're pretty happy with everything that's been done so far,'' she said, blaming her criticism from the US of the search on distance and poor information.
''The news halfway round the world doesn't do justice to what's been going on. We are very appreciative of what's been done so far.
''They [the Marine Police] have been searching for seven days and they've gotten the Navy involved. Now that we're here, we see that the search has been above and beyond what we imagined would be going on.''
Once the pair complete today's search over and beyond the spot where the diver was reported missing about 17 miles off Phuket's Bang Tao beach, they will be heading on to Pattaya, where the diving company Mermaids Dive Centre is based.
Joshua, an IT specialist living in Kuwait and working with the US military, was a frequent visitor to Pattaya, often with Tadsana, his Thai wife of five years. She was also on the hired liveaboard when her husband apparently got ''very drunk,'' according to witnesses, and behaved out of character before vanishing into the early morning darkness.
After talking in Pattaya to others who were on board the vessel, the pair hope to head for Bangkok for an update from the US Embassy ''and see whether they have the police report.''
''They weren't able to give it to me yesterday,'' she said. The hope is that Joshua may have made it to land or be alive on board a fishing boat or some other passing vessel, and yet to get in contact.
His mother said tearfully that the Thai authorities had ''executed the same type of water search that the US Coastguard would have done.
''They've searched a lot and unfortunately found nothing. I just want to see the area and get an idea of what's possible. I hope he's sitting somewhere or on a fishing boat, waiting to be picked up.''
In a statement a few days ago, Mermaids Dive Centre said that after Joshua Devine disappeared, the vessel conducted ''an ocean search for the missing passenger in a logical, planned manner using large spotlights while many members of the dive trip assisted with torches.''
''The dive boat departed from the search area at approximately 3pm,'' the statement said, carrying 22 other passengers plus crew on to their dive adventure off the Similan islands.