If you have a Garmin device that supports open water mode, then you can use any of our swim workouts without a standard pool, including the test workouts.
However, if your spa in indoors, which I suspect, this option is not available to you.
So, you’ll just have use feel and a best guess for an indoor spa. Check out the swim zone calculator at https://www.8020endurance.com/8020-zone-calculator/ and give your best estimate of each zone. Then, when the workout called for “Zone 3” you’ll estimate a Zone 3 intensity and perform the interval for the estimated duration. For example, if you are asked to perform 200 meters at Zone 3, and your best guess is that your Zone 3 is 1:51 to 1:56 per 100, then you’ll swim at a Zone 3 estimated intensity for 3:42 to 3:52.