I’d like to think that while waiting for Arthur, Merlin became a curmudgeony old man, mad at the world and his destiny. He still is waiting for arthur, because he has nothing else to do. He, in fact, invested and has lot’s of money now, owns an estate and many acres of land (which also includes the lake). He hasn’t aged a day, but prefers to stay as his older self because that’s how he feels most of the time. Old. He has also learned that the reason he had an outrageous amount of hair every time he did the spell was because the spell took into account all the years he didn’t cut his hair. He has since fixed that.
And I imagine that when arthur finally came back it would go like this:
“MERBLIIIIIN *GLUBGLUB* HEEERLP!!”
“Arthur, it is literally 4 feet deep there, and I’m not ruining my clothes for you.”