Yes grease should be placed into the cavity of the hub,You don't FILL the cavity, but a small amout behind each bearing and a thin layer in the hub is fine, it serves a purpose, it is not just wasted grease.
I would also use a quality disc brake wheel bearing grease.it will withstand higher temps and will not run out of the bearings when hot. Marine wheel bearing grease is not up to this task.Most boat trailers don't even have brakes and the bearings will not see the temps seen automobiles.Yes it resists water intrusion better, but will you be submerging your axles? The reason boat trailer grease need to resist water is because when launching the boat after a nice highway tow you back the trailer with its hot bearings into the water. The introduction of the cool water to the hot surfaces results in the water being sucked into the hub past the seals washing the grease out.
Do a google seach of "repacking wheel bearings"
you will find many if not the same proceedures on all the sites
From the haynes manual"