I know this is blasphemy, but on my tandem trailer I run passenger tires, P205/70-14 that are rated at about 1300 lbs when pumped up to maximum pressure, which is 44 PSI. The boat/trailer weighs about 4300 lbs. Since they're tandem, I have plenty of stability. Since they're passenger tires I don't have to worry about speed ratings (I never go over 70 mph anyway), and they're considerably cheaper than ST tires.
I wouldn't recommend doing this with a single axle trailer which would probably benefit from ST-rated tires