Finally, i can offer something of value on the forum!
Sorry if this is a long one, but I've trained several pups, of several breeds, to be happy and confident pals along on boating trips...including my welsh corgi who rides along fine in my canoe on fly fishing adventures...but otherwise hates the water.
1: dogs don't have the reasoning ability that you and I do; so for them it's all about confidence and familiarity to be happy. The first several trips to the marina, and on the boat itself, should be all about the dog. Take him or her down TO the boat, give them some treats and love, hang out for a few minutes...then leave. Don't even bother boarding. After that becomes routine, take the pup on the boat a few times while tied up, follow the same treat and love routine, and then leave again. The boat probably shouldn't leave the dock until the dog is very, very comfy aboard..and even then only for a few very short trips at first.
2: Dogs have trouble walking on slippery decking, and unstable feet lead to an unstable dog. Think about some inexpensive indoor/outdoor carpeting , a piece at least large enough for them to have a "safe" place to hang out. In fact , that's a great training tool for where you want them to lay while underway.
3: Take them to a place where you can coax them ( NOT PULL THEM) into shallow water, and let them play some. Get a doggie PFD, and get them used to it. IMO, they should have it on any time while on the boat.
4: a boxer is a less than ideal boat buddy, for the reasons others have stated above. That said, a well trained, confident, and tired boxer can be a great companion. Plan on adding 30-45 minutes of playing, walking, and running the dog on shore before getting on the boat, every single time. A tired dog is a well behaved dog. You can skimp on alot of things, but skimping on running off some of the dogs energy would be something I wouldn't even consider.
A stressed out 70 pound boxer on a boat would be ugly.
Good luck!