The moisture on the bearings and pitting is not from the 100 miles, but from the trailer bearings sitting with moisture in the grease--and on the metal causing the pitting.
There is enough heat generated by just the friction of the bearings to suck moisture in with cold water--nothing to do with having brakes on the trailer.
The bearings will be fine on the way home--but then you should change out the grease. Theoretically the posi-lube/ezy lube systems completely change the grease--it takes about one large tube of grease to completely change the grease (also depends on the type of grease as to the water resistance). So there is not an issue of "over greasing"--you displace all of the old grease with new and water free grease.
I would carry both a full set of spindles, hubs and bearings. If the bearings go, so does both the hub and spindles!
On another vein--I see that you have a single axle trailer, with no brakes under the C Dory. In most states that would not be legal. States and Provinces do not have reciprocity with brake laws as they do with driver's licenses etc. For example some states will allow surge brakes on any size trailer--BC will not allow surge brakes on a trailer more than 6800 lbs. Florida requires that there be brakes on all axles--some states only require brakes on one axle. Many states require brakes on any trailer over 1500 lbs and most over 3000 lbs. Idaho is a bit unusual in that the law considers trailer brakes a necessity on a trailer of unladen weight of over 1500 lbs. I don't know how that is interrupted for boat trailers, but I suspect that a C Dory on a single axle trailer would require brakes--for both safety, legal reasons in other states, and insurance liability purposes. A boat trailer of this size without brakes is considered a "Yard trailer"--not meant for highway use. I suspect that you tow with a big truck--and a few states have a "stopping distance" requirement--but that does not cut it in most of the states. Probably not an issue, unless you are stopped or have an accident.