Or you could just check for moisture around the mounting bolts if you want to take all the fun out of it and do it the easy way.. but you will typically have greater moisture content there that may or may not tell you whether the issue is endemic or systemic.
Did shop 1 guy give you a reason he would not get on the water in it? Did he say there was a specific aspect of it that was unseaworthy? Maybe he didn't understand that it was a boat that is quite capable in the open ocean as WELL as a lake, probably the lack of bow to stern indoor/outdoor carpeting that threw him off.
What is the thing in the last picture you posted underneath the the platform that looks like it would be where a ball check valve/supper would be? The round plastic thingie on the left hand edge?
You can put the batteries in the rear but not an optimal spot for weight distribution and exposure to moisture. In-fact my boat has a original looking glassed in battery shelf in the starboard aft compartment. its vacant because my batt is under the console. For me the salt water exposure has greater implications.