On the minimap a rogue wave icon does appear and shows which direction it is traveling. And to avoid losing crew members you hold the 'X' button(on Xbox), Space on PC, Y(on Wii U), or Square(on PS3) but that will brace the crew and you won't take as much damage.
Also, You can get more crew by going to a tavern, helping pirates in the locations, or picking up castaways in the open sea.
As some have stated in the comments, another way to avoid damage from the rogue waves is to face them head on. But in my experience it is just easier and quicker to brace and then get back to the battle.
Every copy of Assassin's Creed IV: Black Flag comes with a Uplay Passport, giving you access to the Uplay store, which is used to redeem Uplay rewards that can be used in singleplayer or multiplayer mode.
Redeeming the Uplay Passport also gives you access to some of the multiplayer features, such as the ability crafting system, which is available after reaching Prestige level 1.
Every copy of the PS3 version of the game also comes with a voucher that'll allow you to purchase the downloadable PS4 version of the game at a reduced price. That particular voucher has a limited validity, however. In order to play the PS4 version, you'll have to keep the PS3 version of the game disc in your PS4's drive.
The Xbox 360 might have a similar voucher; feel free to edit this answer, should you know more about it.
Exclusively for the PS3 version (possibly also the future PS4 one), there is the Aveline mission DLC.
There is always a chance, that the previous owner didn't redeem one or several of these items, although you shouldn't exactly count on it.
In case your second hand copy is missing any of these: both Uplay Passport and Aveline DLC can be purchased separately. The voucher, however, can only be obtained if not already redeemed by a previous owner.
Best Answer
Whilst having no weapon equipped (fists), Edward will incapacitate the target rather than kill them. This does not count as a kill and you will often see the target rolling around in pain on the floor. They will never regain consciousness.
Using "fists" on a kill target will change the animation to show the neck being snapped or something of the like. Trying to emphasize the death of the target.
There is also some "fist" animations which look like you kill normal enemies. These do not count as a kill though, so will not fail "do not kill" objectives.
There is a scenario in which targets will regain consciousness, but that is when afflicted with a sleep dart. Enemies will regain consciousness after a timer expires which is visible above their heads. Enemies shot with berserk darts will die after their battle frenzy wears off.