Do you need Rum or Sugar for anything

The white skull icon seems to indicate which enemy is engaging in combat with a crewmate that needs saving and is accompanied by a blue dot over said crewmate in trouble, this skull is to help indicate which enemy to focus on to help save that crewmate.

"If you manage to fight through all the Legendaries, apart from the large monetary rewards (don't forget to loot the packages for the money); finishing all four battles will also give the Jackdaw the additional move of a speed ram, where aiming forward and pressing X (PS) and A (Xbox) will allow the Jackdaw to dash for a moment for a raw with increased power."

Here is the complete guide for them: http://m.ign.com/wikis/assassins-creed-4-black-flag/Legendary_Ships