EZ Loader Trailer Hubs

I recently purchased a used boat on a 2000 EZ Loader single axel trailer.
When I went to repack the wheel bearings I found that it had a grease
fitting in the end of the spindle with an outlet in the center of the
spindle. I had assumed it had bearing buddies when I bought it. There was
a membrane over the inner portion of the end of the dust cap. It appeared
to have been solid originally, i.e. no access to the grease fitting without
removing the dust cap, but the previous owner probably punctured a hole in
it to gain access. However, if my assumptions are correct what is the
purpose for the membrane? When I broke the hub down it was full of water
with very little grease and very rough bearings. The paint seal over the
dust cap had never been broken so the bearing had never been repacked. I
bought the boat from an individual through a dealership. At my request the
salesman pumped some grease into the fittings before I left. If he had not
I would have probably have lost a wheel on the way home. Question for
anybody who has experience with this type system: How good is this system
at keeping water out of the bearings if kept greased? How should I maintain
it? Thanks in advance for any info you can provide.