Sunseeker =charles is the son seeker. If you understand the comics and the movie charles is a mentor and like a father to all mutants and treats them as his children.
Hence why he is excited about more children being born.
Because they are like his kids and he knew how many mutants were left (not many). So he was happy about this.
But ever since his ALZ. He hasn't been able to be a father /mentor.
So he is the the son seeker(sunseeker).
And he was searching for a sun in his last student(logan).
Thats why the whole pop and my dad thing made it obvious.
And when he says its got water!
It means the symbolic means that charles is the sun seeker, who taking in water and sinking or sunk.
Also taking water in the brain is another form of Alzheimer's. Its an old saying. But saying "he has water in the brain" is a reference to ALZ
So taking water in is what happend to charles.
The water has a few meanings.
Hope this helped:)