I don't know anything about those trailers. But when I'm looking for a trailer to drop in the water there are two areas I look at closely:
1) Wheel hubs - Wheel hubs must have a grease fitting that is easy to use and pushes grease from the inner races outwards, which is what pushes water out. these grease nipples are typically found on the centerline of the axle and on the outer race side. There also needs to be a drain hole in the hub. You just won't ever get these to be completely sealed so water needs to be able to drain out easily.
2) Lights. The lights do need to be completely sealed. I look really carefully at all the seals and the light body to make sure it's completely waterproof. Especially check the lens for the license plate. those are often overlooked.
Aluminum frames are lighter and tend to be less prone to corrosion in years to come. If it's a steel trailer, make sure it's heavily galvanized, not just painted.
dj