Good story, bad effects and bad science (spoilers)
I liked the story (and I'm surprised at the lack of discussion here) and it was somewhat moving, but I feel it was held back from being a great film due to the mistakes in the effects and physics. I think it deserved a bit more than that.
It's clear that whoever did the rainbow effect has never seen one. I mean really looked at one. The sky is slightly darker on the outer (red) side of a single rainbow. The outside rainbow in a double rainbow is in reverse order to the inside rainbow, with slightly darker sky in between. I mean, for a live-action film with "rainbows" in the title, you do actually need to get it right. I'm not aware of any way that a person could be inside the colours of a natural rainbow, let alone be seen to be at the end of that same rainbow by an outside observer, but I'm ready to be proven wrong.
The mechanical seal pup did not move in a natural way, very robotic. In the age of Gollum, clearly we can do better. Also, while that pup was given food, it didn't appear to ever get any water to drink, being stuck to that one spot for nearly the whole film, a period of perhaps weeks, sometimes untended. Are we to assume that the tides took care of this? Also, the underwater sequence with the seals and Tomás didn't look quite right either. It looked a little like drag-and-drop on my desktop. Fortunately it was quick.
I wish just a bit more care or expense had been taken to get it right. The story is worth it.