Should Add? - Single Choice
Dolphins are already slated to be added in 1.13/14.
So are fish, fish (or at least turtle food), sea armor (turtle shell helm things at least)…
Tanks are apparently intended to be multi-block constructions created by players.
All the listed "things" in the OP need more extensive detail to be useful suggestions (eg. what is a sea sword and how does it differ from current swords)
WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
You have provided no information on any of these items. We can't read your mind or create ideas for you. As a suggestor you need to preset your ideas to the critics (us). Right now there's nothing to support.
Nether Progression, Elephants, Creative Inventory, Chorans, more