Yet, that is precisely what Eph 1:13 does say.You can't just say every believer is sealed with the Holy Spirit.
Ephesians 1:12-14Revised Standard Version (RSV)
12 we who first hoped in Christ have been destined and appointed to live for the praise of his glory. 13 In him you also, who have heard the word of truth, the gospel of your salvation, and have believed in him, were sealed with the promised Holy Spirit, 14 which is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory.
The passage tells us how one is placed "in Him". I prefer the NASB:Again we see the words, "In him". In Christ. So the words are for those who are 'In him'. Again, if he is not in him, then he has no inheritance.
"In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise,"
So, having believed, you were sealed in Him.
They are still sealed FOR the day of redemption (Eph 4:30). There are no verses that teach that any believer will be "unsealed" for any reason.Many believers will fall away.
I'm glad you mention "the living water", because Jesus also mentioned living water.But let's assume a man comes to life in the Spirit, but he does not remain in the vine. This man can no longer receive the living water, and so he withers and dies.
"but whoever drinks (AORIST TENSE) of the water that I will give him shall never thirst; but the water that I will give him will become in him a well of water springing up to eternal life.”
By using the aorist tense for "drinks", Jesus was noting that he wasn't teaching that one must continue to drink in order to have eternal life, but that from a drink in a point of time SHALL NEVER THIRST.