Yes, they were designed for marine use, but a lot of trailer manufacturers use them, so this question comes up a lot.
The Dexter manual says to inspect the e-z lube bearings every year, which would mean you would re-pack them at that time, and replace the rear seals. However, many people get away with only doing it every two or three years, especially if you don't put on a lot of miles. If you do "lube" using the nipple, make sure you raise the wheel off the ground and turn the wheel while injecting grease - failure to do this seems to be the major cause of the blown rear seals that so many people talk about. We don't use our trailer very much, just two or three thousand miles a year, and I do an inspection and manual re-pack every two years - it's not that big of a job, once you learn how to do it. I don't bother to squirt in any grease between manual re-packing, but if I ever feel the hubs getting too hot to touch, then I will.
Hope this helps.