Marine grease if you dunk the axle in the water. I Work on trailers for a living, but mostly cars and pickups. The biggest thing I can say to anyone is pull them apart and pack them every year. Also, bearing buddies are a good idea on No-brake axles.
Here is why I say that: Bearing buddies are spring loaded dust caps that you fill with grease until the spring has load on it, causing positive pressure in the hub. This pressure helps to keep water from entering the hub and ruining parts.
You can add too much grease to these, either causing grease to leak out the seal or blow the seal out. About all that does on a no brake axle is make a mess, and you may have to replace the seal and do a bearing pack. On a braking axle, this typically contaminates the brake shoes and they have to be replaced too.
I would never assume that bearing buddies replace regular maintenance. (yearly for those who dip their trailers) It is a simple thing to do, and trailer seals can be found for cheap.
Wishing I was on the river instead of surfing the web...