It's been proven that when people have to wait, they perceive it to have been 23% longer on average than it actually has been. So it might not have been quite as long as it seemed that the guy was gone. Also, he might have gotten up there and noticed the damage to the other two trucks and spent some time inspecting his own to see if it had been tampered with, which could also explain why it seemed he was gone so long.
Bottom line is, that could have happened at anytime during the day and also to other people who returned and had the same thing happen but left before you ever found out about it happening to you. It could be a coincidence that that guy was returning at the same time as you and his truck wasn't touched. If you're going to steal, would you waste time trying to get into a truck that didn't have anything in it to take?? Maybe they saw his had nothing. There are a dozen ways to look at this, and none of it makes the guy guilt free but definitely doesn't prove he did it either, nor does it prove anything at all except that two trucks were tampered with by somebody at sometime during the day.
I wasn't there, so I don't know if he had anything in his truck or not, or if you did either. So don't take this as me trying to prove something. I'm just offering other ways to look at it from a 3rd party, unbiased person with a clear mind on the matter, able to see it in ways that you may not think of for being upset by it all.
Having said all that, I'd get away from that guy totally. Leave him as a friend on Facebook, so he doesn't suspect. You can also monitor his posts that way too. Maybe you'll know when he's going to the lake, or has been. Also, speak to him when you see him in passing, so you dont become a constant target of his. But riding to the ramp with him, fishing with him, and other outside contacts, I wouldn't be caught dead doing that. Anyone with that record, I would not associate with at all, ever, as they are beneath me and not of the same caliber of person that I'm trying to be. I'd also be aware of him in the future as well, that even while being friendly to your face, he would still steal from you behind your back. Maybe next time, he'll get caught by you and then it can be proven if you are aware and do not let your guard down.
As to what you might have done in that situation that night?? There were two of them so it was stay with the boat and let the first guy attempt to vandal your truck, or tie off and go to the truck and risk the second guy stealing from your boat. Bad situation. Unfortunately, the answer is to have been off the water before dark and maybe not even going alone to begin with. People are sometimes targeted based on how easy they have made themselves to be targeted.