Are you sure it is the tires? Sway could easily be caused by improper balance of the trailer. How much does the trailer weigh when fully loaded? How much tongue weight do you have? It should be about 10-15% IIRC.
BTW, did you stow the gear on the boat differently or add any equipment to the boat between last season, when it worked fine and now. Try shifting some of the contents of the boat forward in the boat, and see if that affects the swaying. If it does, then the issue is more likely a trailer balance issue than a tire issue. Even the addition of a single battery for your house bank, in the stern of the boat could shift the balance enough to alter the tongue weight drastically, since it is at the end of a relatively long lever arm...
Trailer tires have to function under fairly extreme conditions, and if ignored, they tend to fail rather dramatically. They're generally narrow, small diameter tires inflated to much higher pressures than car tires are. Given the small diameters, they have to rotate much faster to go the same speed as a larger diameter car tire, so bearing condition and tire pressure are more critical to maintaining their ability to do their job.
A tire from any of the manufacturers that meets the size and pressure requirements of your trailer should work relatively well. I would look at Consumer Reports, BoatUS's Trailering Magazine, or one of the trailer boat specific magazines, as one of them probably has done some tire reviews.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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