Hi tommyhip. If rowing intrigues you, let me offer some additional information for you to consider. I think that there are a few options for you to consider:
1. Rowing shells. These are a great whole body workout, and also involves the legs which you may or may not want. The seat slides on a track powered by your legs, so each stroke with the oars involves legs, core, and arms. These are very fast through the water. Speed equals long and narrow, so these are often 20 to 24 feet long, which may be an issue for you. Mostly restricted to calm water due to the narrow tippiness.
2. OC1. Which stands for outrigger canoe, one person. We see these a lot in Hawaii, since the outrigger allows for use in very rough ocean conditions. Uses a single paddle instead of oars, so you must be sure to balance your paddling on both sides.
3. Surfski. Long and narrow, this uses a double bladed kayak paddle. One advantage is that your are facing forward, and the double bladed paddle is more efficient than a single paddle so that keeps your workout balanced on both sides. Typically 16 to 20 feet long. Requires some balancing skills.
4. Stand-up-paddle boards. Great upper body workout, especially core. Since you are standing, there is some workout for the legs, especially on the faster narrow boards which require substantial balancing. If you go this route, I think you would be best served on a narrow race style board, which would b 12 to 14 feet in length, and 24 to 28 inches wide, depending on your balance skills and weight. I would beware of buying the usual boards that you will see at Costco or other common sources. Most of these are for families to play around on, and are slow and cumbersome on the water. If you are going to spend any time for cardio workout, you want a sleek board that will be fast through the water, though it will not be as fast as any of the above craft; just a lot easier to store and transport. Some brands to checkout would be SIC (the brand I use), Bark, Infinity, Fanatic.
Personally, I find the water to be a wonderfully peaceful, quiet, and beautiful place to get into a cardio zone.