There is no road leading to Dolphin Bay Divers Retreat. Trust us, even though your map might say otherwise. So renting an SUV is only going to get you to Buca Bay on Vanua Levu or to Taveuni.
Bligh Water Shipping has 2 ferries, the Westerland going from Lautoka to Savusavu or the Suilven going from Suva to Taveuni via Savusavu.

From Savusavu you can either fly to Taveuni or take the Raja or Grace ferry to Taveuni (via bus to Buca Bay).