If you want to observe and enjoy the sea life, you will need a mask with a wide field of vision.
Single lens masks often provide a wider field of vision.
The comfort and the ergonomy of a mask rest on the skirt, this is why Decathlon's masks have 100% silicone skirts. Consider using a neoprene strap if you have long hair or for children.
An apnea mask has to have the smaller volume possible.
2-lens masks have a smaller interior volume than single lens masks.