government resthouse
There's a government resthouse signed only in Bengali on the main road next to Bay of Bengal Guesthouse. Rooms are a little larger and cleaner than the surrounding guesthouses and have mosquito nets. It's not possible to prebook rooms and, even if they're available, whether you can stay will depend on the mood of the man on duty. Rooms ~Tk 600.
There are several budget guesthouses along the main road such as Hotel Sea Heart, Bay of Bengal Guesthouse, Coast Guesthouse, Hotel Sadek, Oceanview and Coral Reef Guesthouse, all similarly dark and characterless with dank attached bathrooms and no running water. Anywhere else these rooms would cost around Tk 500-700, but here you'll be lucky to pay Tk 1000 and be asked for as much as Tk 1500.
Most evenings will be candle-lit since there's no electricity on the island, but a few of the more expensive hotels have generators.