This is a problem with us as well. We've got a 90 pound yellow lab that loves to go boating with us. We are lucky that we back into the beach so we're usually waist deep in water and it's just a matter of lifting his front paws onto the platform and then pushing him from the rear!
There are some dog ramps online but they do take up a fair amount of room. There was a couple we met at Norris Lake last year. I can't remember thier names but they had two dogs with them. The dogs had life jackets with handles on the top. When they wanted to get into the boat the folks would just lean over from the swim platform and pull up on the handles. Worked really well. And those would take up less room than a bulky ramp.
We'll probably go the life jacket route eventually.