Rohan is not known for their sea-warfare.
After rumors of that the ally Gondor use "tree-house" on water to move.
The scientist Aldor Grimsson at Isengard "science center of stolen intellience and pathology" put a group of laborers to build a tree house which can move across land and sea. Aldor Grimsson simply put a tub full of water beneath the tree-house. But it didn't work and Aldor drew conclusion that magic was needed...
The evidence