Boat cushions are interesting in they are all compound curves. That means they are cruves upon curves.
You can not recover them yourself... Nor can a furniture reupholsterer because a lounge suite or armchair isnt a compound curve either.
You need someone who is an expert in compound curve work... And theres only two trades that do it... Marine trimmers who are very expensive... Or, as Stumble says, a car seat reupholsterer. They work at the right price for a market too scabby to buy a new car seat.
Third world countries have lots of them because vehicles are kept longer than the seats last, mini-busses in particular.
I had mine recovered in microfibre in Malaysia when we were drifting through. It did wonders to brighten up the interior and the cloth unzips and washes clean.
So find a car upholsetery place that does not work on Ferraris but works on old heaps of junk....
Or wait till you get to some largish third world country... Dominican republic, Cuba, asia etc