In the Seafarer's Expansion for Settlers of Catan, is the Road Building Development card applicable for ships?
Yes, the Road Building Development Card can be used for ships or roads.
From the Seafarers rules, page 5.
When playing the “Road Building” Development Card in a game of Seafarers, you have the option of building 2 roads, 2 ships, or 1 ship and 1 road.
The "Road Building" progress card in Cities & Knights explicitly states that, when playing with Seafarers, you can play either 2 roads, 2 ships, or 1 of each.