I recently purchased a 20’ pontoon boat with a dual axel trailer equipped with surge brakes. I am planning on towing a jet ski trailer (single) behind the pontoon (tandem tow). The jet ski trailer does not have brakes, I am looking for an anti-sway device for the jet ski trailer. Do you have any suggestions.

Since no one has written of their actual experience, I would suggest you try the tandem trailer without the anti-sway. Remember that the trailer balance has a great effect on stability. You'll want about 10% of the combined trailer & jetski weight on the hitch. If you still have sway, check the tire pressure on the pontoon boat trailer and its hitch weight balance. Then consider a friction sway bar for the jetski.