Having destroyed more than my share of utility trailers off road, I'd say bigger tires for sure. That suspension is already a huge help, so that's good. Those fenders will either have to be removed or raised/replaced. You'll need them on the highway, but off road it's best not to have them. Maybe removable ones? And of course, SLOW is how you keep it from falling apart.
For getting them out of the water, I second the front hitch. Easier on you and you won't back up the ramp trying to muscle them out.
The covers. I wouldn't trust them at highway speeds. Can you install grommets and then bungie them under?
