Why did they leave?

Here are my thoughts after watching the movie for the first time last night. [After reading the comments here I would have liked to scan through the movie again but when I checked my mailbox just now the postman had already picked it up to go back to Netflix. :( ]

Anyway, I admit I can't remember the funeral scene at the beginning very clearly but it seems to me that everything (except for perhaps that) in the movie points to Fiona and Jamie's mother being a Selkie. I think the story told by Fiona's father about how he met her "travelling around" might have been a tale to cover up the real truth. Notice how he brought her back unexpectedly in a boat just as had occurred in the story Fiona had been told of the earlier selkie. No one from Roan Inish knew Fiona's mother before that. I'm thinking she didn't actually die, but rather found her skin and returned to the sea. We never do get details about what happened to her. She might have been "lost at sea" and her body never found even though they had a funeral for her.

I think Fiona's father's reaction is consistent with (but not proof of) that. He knows what he's lost, just as the earlier selkie's husband knew what he had lost.

Also, I don't think it's coincidental that that one particular seal has a bond with Fiona (and vice versa). That seal notices her on the boat when she arrives and seems to follow her for the rest of the movie. I think that seal is Fiona's (and Jamie's) selkie mother, checking up on her daughter.

In regards to this from Myndchanger's entry:

"After awhile his father noticed that the cradle had floated out. The father and uncles went after it but for some reason couldn't catch up. Jamie was lost to the sea. Supposedly."

I don't think any of that was coincidental either. If you watch the movie you'll see that the cradle didn't just float out to sea. It floated out to sea in the midst of the bird attack on the men. The birds were distracting the men on purpose, and there was one brief shot of a bird actually tugging on the cradle (as far as I could make it out). The birds were helping out the seals, i.e. the selkies, i.e. probably Jamie's mother. Notice how dark Jamie's hair was (then and later in the movie, just like a selkie's)? And notice how that bird keeps eyeing Fiona. The birds are part of the tale, too.

I think the reason the men couldn't catch up with the cradle, even when rowing at full power, was very specific - a selkie (again presumably Jamie's mother) was towing it away. That a selkie or seal could do that was shown quite clearly later in the movie when Fiona is towed over to Roan Inish in the boat without oars.

So my overall take is that after Jamie's mother turned back into a selkie, she missed her children, and took the opportunity when it presented itself (with the help of her seabird friends) to "kidnap" her young baby, not out of an evil instinct but out of a maternal one. She wanted to be with him. And the fact that he survived all that time in good health showed that she had cared for him. But in the end, her maternal instincts also told her that he needed to be with people to be raised properly, and at the end of the movie she saw to it that he was returned to his human family. That is, after she knew that he would live on Roan Inish and be nearby where she could keep an eye on him from time to time.