I think our experiences with Chester were very similar to yours
@RightStuff . He loves the wind blowing by his ears too! lol When we first get to the boat ramp, he of course is excited. So much so, that we have to make sure we aren't, so that he is calm. But even so, when I put him in the boat, all 70 pounds of him, he is all over the place. I do so from the ground into the boat on the trailer, so he is up on the seats and waiting eagerly for me to get in. I too attach him to the window support once I am in the boat, but the longer lead just had him up in the seats and going side to side waiting. So I shorten the lead to keep him on the floor and center boat. Once we are away from the ramp area and underway, I lengthen the lead. After 10 minutes or so, he is calmed down and has his water legs back, so he isn't restricted to the lead. He has never offered to jump from the boat, so I don't worry too much about that, but he is a boxer, and not a water dog like labs and retrievers. I think a little rope running across the seat backs with suction cups to hold it, would prevent them from reentering the boat when you don't want them to, restricting them to the swim deck...just an idea. Because chester goes #1 on the swim deck for us since he was a pup. But his ritual is to run around anxiously until we tell him to go, it would be easier to just tell him to go potty and open the rope across the back like a door, and close it behind him. Then he could take his time intend of daddy prompting him to go potty!
One thing that I have noticed that is a big issue I think is jumping from the swim platform. They don't know the depth, and they get used to jumping in like the water is deep and then it is only inches. I worry they could break a leg easily. So if you can keep from having them jump, I think you might be better off in the long run. That may not be an issue where they can see the bottom but many of our lakes are not that clear, and I see chester pacing, unsure of how he should get off the boat. I have seen him hit hard too, and scared the crap out of me! Anyway, dogs on a boat...there is no other way!