"Players may not use TWO OR MORE clients at the same time to PvP."
None of these boats/chars are moving at the same time, which makes me believe it is one person multiclienting.
I've always taken it to mean simultaneous actions by multiple alts (all 3 attacking you at once). If he is using alts one by one (brings boat in, brings another boat in, brings galleon in) then it might be legal, who knows.
Here it is. Again, from http://www.uoforever.com/rules.php
"Simultaneously playing MORE THAN ONE SINGLE ACCOUNT at any time is prohibited (Moving on TWO or MORE accounts at any given time). "
That's not PvP. That's ever at all.
What if he came along when all 3 of my fishers, who are archers, are fishing for serpents? I can legally use all three of those archers to defend myself...in fact, they will automatically defend by shooting arrows once attacked.