Following a mention from Danny that it is possible to open the Navman and dry the insides out.
Well... Couldn't resist so I took a torx driver (T6) and opened it up
and left it to dry out. Had a quick look at the keypad membrane which is the most likely place water is getting in, but couldnt spot any obvious ingress. It seems well designed with rubber seals all around. Maybe it was humid in the factory when it was made.
I had a tiny sachet of silica gel which fitted perfectly so should keep it dry.
Will let you all know what happens. Am hopefully out this weekend up the Thames.