I wouldn’t mind having the mobs tell part of a story, You spawn on a stranded shipwreck onto uncharted lands, where you have to go and survive.

1. We could have groups of “bandits” or rebels, maybe small pockets of Pirates that were shipwrecked and have setup camp somewhere (May be better as NPC’s and have quests)
2. A sea based mob, sharks, Guardian/Squid. Squids could be used as sharks and re-textured in the custom texture pack as sharks.
3. Hostile mermaids/Sirens that pull you towards them and drown you, holding you underwater, could only be found around certain parts of the map.
   
4. British/Spanish soldiers, could also be found on certain beaches with their ships not far from shore, maybe protecting some treasure on a beach? Could be a way to fight them for loot, these areas could be generated on a time basis so there not always accessible.