Hello from Reunion,
in Reunion there are some youth hostels and backpackers.
The islands capital (St.Denis) is not really the most interesting place to stay, but still ok for some 2-3 days.
Most people go to the west and south, around St.Gilles, St.leu, St.Pierre
There's a campground at Etang Sale les Bains. On many beaches wild camping is tolerated in the holidays (you will find quite a lot of locals camping in St Leu, Etang Sale, St Gilles. There'd been a campground also at St.Gilles Hermitage Beach, that is supposed to reopen this December (if i'm not wrong)...
Backpackers hostels I know from St.Paul (great market on Friday/Saturday morning), St.Leu, and St.Pierre.
In St.Denis I don't know any backpackers - but there are some kind of cheap pensions where mostly people from the neigborign islands stay (Madagascar/Comoros, etc). I suppose, they are quite effordable.
Youth hostels are found in St.Denis, somewhere in the higher parts of St.Paul (I think Guillaume??) and somewhere in the interior (??forgot it)
You may also take a chance to search after "Auberges" that's mostly some rural families that rent rooms.
Some of them are affiliated at "Auberges de France" (they got a web-side somewhere...)
The catamaran ferry had stopped service already a few years ago, due to problems with the sea conditions. That ferry was not suitable for open sea (it was a high speed catamaran, build in Norway, more likely to operate in calm Norwegian Fjords).
The Mauritius Pride and Mauritius Trochetia (both are of the same company) are the only passenger vessels between the islands- prices are almost the same as for an air ticket.
Should be reserved long time in advance... they are mostly booked out a few months before departure, mainly at school holidays.
Same for flights during the school holidays in Reunion.
Those vessels also serve Madagascar (from Reunion) and Rodiguez Island (from Mauritius).
They are mixed cargo/passenger ships.
Public transport in Reunion: there are buses all around, every city/suburb is served.
Between the main citys every 20 to 45 minutes, some suburbs only 4-5 times a day.
Every big city got a bus terminal, that links the suburbs and the interurban buses (car jaunes).
Direct bus from the airport to the central bus station of St.Denis is available.
As it 's a little more expensive than regular buses, you may try your change in heading to the motorway a few hundred meters off the airport, where there is somewhere also a bus stop for the regular services (yellow buses, called "car jaune").
Latest departures of the "car jaune" ist mostly around 6pm - 7 pm !! depending on the city.
Hitchhiking is possible too.
Surfing: best spots around St.Gilles, St.Leu, Trois Bassins, St.Pierre, but also in many other cities. All year round
there's a lead of the world surf championship in St.Leu once a year (known for it's famous left wave)
Anyway, you can't miss the spots, most can be seen from the motorway.
Food, eating, etc. : more or less a little expensive. Cheap food at most "Camion Bars" = Snacks that you will find almost everywhere.
Most offer meals, drinks, salads &sandwiches all day long. In the evening you will also find "Camions Pizza" in evey town for take away Pizza (many, mainly at the beachfront, also have some tables/chairs)