Does anyone have an understanding of what the sea in Mark 11:23 might stand for?
It represents nothing.
The entire passage is simply a way to give the apostles some concept of the MAGNITUDE of the power that was available FROM GOD, when his WORD to them was TRUSTED absolutely - i.e. when they had "FAITH" in Him.
Jesus offed a fig tree, and the Apostles were AMAZED. Jesus essentially says "that's NOTHING compared to the true extent of the power that God has, and which is potentially delegated to YOU - if your FAITH is in What HE's communicated to you".
It's the Fig tree itself that is more significant typologically.
The tree LOOKED as though it had fruit - but had none. The story probably ties in with Luke 13:
6 He spake also this parable; A certain man had a fig tree planted in his vineyard; and he came and sought fruit thereon, and found none.
7 Then said he unto the dresser of his vineyard, Behold, these three years I come seeking fruit on this fig tree, and find none: cut it down; why cumbereth it the ground?
8 And he answering said unto him, Lord, let it alone this year also, till I shall dig about it, and dung it:
9 And if it bear fruit, well: and if not, then after that thou shalt cut it down.