Those wheel nuts might have been tampered with - sad, but things like that happen these days.
Liquilube is supposed to be the business, but, as pointed out, you might not get it to fit UK/European hubs.
Spring loaded bearing protectors. I tried these years ago, and did not really get on very well with them. It was too easy to get grease leaks from the rear seal. Then one actually fell off on the motorway - could have killed somebody! Its quite easy to check normal bearings and give a quick injection of grease if needed. Take bearings out once a year, clean and repack. Its not a difficult job once you have done it a few times. Trailers used for freshwater launches should not need repacking more often than that. Its not like you are beach launching and getting silt in there.
Let your bearings cool before launching - that will help stop water being sucked in.
I run AlKO sealed bearings on my trailer. Just got 8 years out of a set.