My daughter is two right now and was raised aboard since birth. We had a similar issue of course. I used the car seat method until she was just too heavy and the car seat just too big. I invested in a BabyBjorn front harness and took her EVERYWHERE in that. On and off the dingy, up and down ladders, docks, ramps, grocery store, walks, you name Lucie was in it. When I was in a situation that I thought needed a life jacket, I put mine on. The Bjorn is a front carrier so that wasn't an issue. If something were to happen, it is very quick and easy to get the baby out. I did trip on the dock
one day (didn't fall in) and the Bjorn worked very nicely in holding my little one securely to me.
This might sound strange, but I didn't feel comfortable having my little one in a lifejacket. Even the smallest one was immensely too big and bulky, I couldn't get a good hold on her when she was moving about, which I felt was even MORE dangerous. When she started walking on the dock, I would put her in the lifejacket mostly because I could hold the back handle and keep her from falling on her knees. She grew up learning
to walk on the dock and has never needed to wear one in case she falls off. She just knows that you need to walk on the dock and not in the water - normal for her, but I suppose not for others.
I have also started a website called Kids4Sail.org. Take a look and join up to find kids out there on boats.