The earlier the better. I have two labs (7 annd 3 now) and both started in the boat as tiny pups and started hiking around with me at about 6 months. My advice is to go somewhere where you aren’t likely to be around other people. That was the most distracting thing for my young labs and still can be. Bring treats for good recalls if he starts to get too far away for comfort. The toughest part for me when they were young was the whining when I waded into the river. Both would sit at the waters edge wondering why we weren’t playing fetch and just whine at me. Ignore it and stay focused on fishing, and they’ll learn dads busy and start to entertain themselves. I let both of my dogs play at the waters edge but taught them what the boundaries are pretty quick. Definitely no swimming unless I throw something for them. Honestly they figure it out on their own, the river is the greatest dog jungle gym on the planet. Sticks, rocks, lots of sand to dig in and animal sign to sniff. They stay busy and at 13mos old he won’t get tired or want to quit. Just make sure he comes to you when called, the treats will help.
The boat is another matter. That can be pretty overwhelming for a young lab at first and it’s best to make the first few trips with no rods while you see where you are at. He will have to learn it’s only ok to jump outta the boat when you give him the command. But the more you do it it’ll sort itself out quick, they are smart dogs and want to please. Have fun!
Be cautious loading and unloading at the truck, it took awhile before I trusted them off leash all the way to the rig. A dog with the zoomies along the highway is nightmare fuel. Bring a leash until you can trust him.