When my wife and I had our first boat we bought two Dahon Mariners. Dahon is one of the largest folding bike manufactures so the price was reasonable. As well the Mariners are aluminum framed. What I noticed under our 200 lb body weight was that they flexed a good bit under strain. Not enough to make me not want to ride them but enough that I would only ride them on fairly flat conditions. ie A boardwalk, in town to run to get groceries, etc.
If I was to buy another folder, which I am considering, would be the Bike Friday Tikit. It is one of the best folders out there as far as I am concerned. I have talked to owners, nothing but raves. Best warranty out there and made here in the US. Second pick would be a Brompton.
Cheap bikes are just that, cheap. Not fun to ride, always need adjusting and plainly don't serve the purpose as most don't use them. Think of it as an extension to the boat, the great thing about folders you can take them with you in the car to work. I took mine to work, stored it in my office and road it during lunch.