For the use you have described, get a greaseable bearing protector. Bearing buddy is the industry leader.
https://www.bearingbuddy.com/ I can not tell from the picture if the trailer hubs have these protectors on the hub end.
This is the reason why …... trailer the boat to the water. Back trailer in and put boat in water. Pull trailer out and store. The bearings were very warm from the tow and you dropped it into relatively cold water. Inside the hub everything contracts and maybe pulling in water. Then you park the trailer for a long period and that water has the opportunity to do a chemistry project on your bearings. With a bearing protector, there is a seal with spring tension at the hub end. The spring will move out and back in with the temp changes keeping grease in and water out.
To put grease in there initially with the bearing protector is a Goldilocks thing. You want enough grease in there so the spring against the seal has something to work with. But not too much that when things heat up the grease pushes out the rear seal or pops the bearing protector off when towing.
This will give long life to the trailer bearings in a not so nice environment. There are better things to do in life than replace bearings and races that have pitted and then ruined because water got inside.