Concerning NPCs
1. NPC's that arrive on small rafts of their own, males, females, quests that determine their roles or how they interact with you. Different ways in which you can be threatened by npcs. Pirates.
2. Random encounters with npcs where as soon as you turn your back on the nice guy as you go get the items he's looking to trade, he stabs you in the back. Or that friend you made finally got sick of your lazy ass, and has you for lunch.
3. Encounters with small raft villages/outposts that provide quests, trade opportunities or info concerning possible dive sites, other outposts or areas to avoid. Some allow you to join, others are threatened, give you a limited time in the outpost. A bartering system that effects attitudes.. the town trader or people drinking at the bar flips out causing a fight, landing you dead or incarcerated.
Concerning game play - this reminds me of Water World.
1. Add a diving system, dive down into sunken cities collecting trinkets that could possible be dismantled for parts to be used for more intricate crafting. quest items, deep see spear fishing.
2. Wind, and a way to determine your direction? Such as with a compass or using the stars,so you can sail to different locations, wind turbines, power generation. anchors. boats?
3. Hunting and a better fishing system. I've always wondered what shark fin soup tasted like. Make it a challenge, different gear for different fish, make it interesting.
4. Items. How does that lantern stay lit? bio-fuel? whale oil? oils left over from cooking that fish? Gas or fuel, hoarded by groups like the smokers in water world. Where does the dirt come from for those plants? the collection or dirt and possible rocks for growing or building ovens, kilns or forges. Blacksmithing from salvaged items.
Just a few thoughts..