Am I blue? You would be too…

My understanding is that it is mostly practical.

The interior has to be some colour. The fibreglass is naturally white, but white would be very hard on the eyes because it is so reflective of the tropical sun.

Making it dark would reduce reflection, but it would absorb heat making the surfaces burn the skin.

They want to be able to see the fish that have been caught, so any similar colours are out. And they want to avoid red/pink because they want to be able to wash out blood.

Blue fits the bill to address all of that, plus it is easy on the eyes that are already adjusted to the blue of the sky and water.