Not being able to board ships without first weakening them. How cool would it be if you could stealth swim onto a big ass boat and assassinate everyone onboard?
I did that once and just started murdering everyone on board. Eventually I got sniped by the crow's nest guys. It was weird because it didn't trigger a battle or anything (music didn't kick in, my crew didn't start boarding) so I don't know if anything would have happened if I'd managed to stay alive and kill every last one of them.
I've had mixed experiences with this. One time, with a ship I'd weakened with my own but was near another ship I didn't want to fight, I snuck on board and killed everyone. Then I did the same with the unweakened ship. After swimming back to the Jackdaw and hitting the board prompt, there was no fight on the first ship, but crew people did appear in the cutscene. In the second, I just shot it once and it became boardable, whereupon the same thing happened.
Today, I tried doing that with another ship. This time, I fast travelled away from the now empty ship and set a marker where it was. When I returned with the Jackdaw, there was no ship there.
So, it seems as though you can totally assassin your way through any ship rather than doing the dangerous boarding exercise, then board after that, as long as you do not load in between.
I love that that is all possible.