I'm in Ecuador and I want to go to Galapagos. As you know, it's expensive to go there (no matter by boat or by plane). I wonder if there is no possibility to hitchhike a boat (marine, cargo) from the continent to Galapagos. Does anybody have experience in hitchhiking to Galapagos?
You can't hitchhike to the Galalpagos.
If you are trying to do things on the cheap your best bet is to turn up to Guayaquil in Ecuador. From here they have boats cruises that depart on tours around the Galapagos. If you have time on your hands you can hang around and wait for a boat tour where they haven't sold all of the spaces. Depending on demand, you can get a deal for as much as half the price of a normal ticket.
The other option is to pay a cargo boat to take you. It can be a bit of a rough ride though and you may need to bring your own hammock. I believe it takes about 3 and a half days on a cargo boat so it probably isn't the most comfortable!
One thing to add to Kizzle's answer is that Tourism to the Galapagos is highly regulated. The number of visitors is limited and all tours visiting the area are counted, registered etc, since 98% of the land is national park. You cannot visit those areas by yourself, you need a certified guide.
So even if you own a boat, you cannot just travel around and visit the islands.