Unfortunately I cannot bring up the deck plans for the Dolphin deck, but the only cabins showing "shower only" on Main deck are G 2504 & 2505 at the very front of the ship and G2713 & 2715 at the very back of the ship. Wheelchair accessible cabins only have showers. All the other cabins on this deck have a bathtub with an overhead shower.
Can't answer your question about rowing machines, but someone else may know if they have them in the gym.
Personally I would choose Main deck over Dolphin.