I used to grab both hands on an elliptical trainer, and it provided a surprisingly good training for SUP paddling.
Imagine grabbing one of the "arms" of the machine below with both hands in a SUP paddling position. The benefit is also that you can monitor your cardiac rhythm and thus get used to recognize when you are reaching the cardiac danger zone on the water.

You can also see a lot of dedicated SUP rowing machines, adapted from kayak/canoe machines. Google for "SUP trainer". Here is one, but there are many others:
vasatrainer.com/product/sup-standup-paddle-ergometer/
But, I have not tried it, but Jim Terrel pool system seems the best solution. Either put a stool in a kids inflatable pool, or stand on a bench with 2 rectangular basins on the side (horse water trays) filled with water, and paddle in them.
Or just stand on the side of a pool: