I have a water rower rowing machine and I had a quick go at using it as a standup paddle trainer.
You just get about a metre of dowel and some rope or webbing to hook it on to the normal rowing handle and away you go. The resistance is quite strong and it feels a bit like paddling a super heavy board. I guess you could use any rowing machine or even a weight on a piece of cord suspended from a pulley.
The paddling worked quite well in terms of working the muscles but switching sides was a bit awkward. My machine is not real happy about being pulled on an angle so standing in the middle and switching the paddle is not real good. It's best to stand to one side and pull the paddle straight through.
If you do paddle sets on alternate sides and spend the few seconds to move yourself over and swap sides it is quite doable.
I guess for the cost of a length of dowel and a loop of rope it is worth doing. It makes a change from sitting and rowing and you can do it when access to the water is not possible. Throw on a SUP porn DVD and it could be a good way to get some exercise. Add an indo board and somebody throwing buckets of water over you and you have a SUPsimulator.
