They are both primarily about solving a mystery as you explore different locations.
EO1 is fairly linear. You pick a spot to dive, explore and find new fish, try to find new locations, and get some lore. Pretty relaxing.
EO2 has a more involved story, that takes you to different places around the world. The Aegean Sea, Canada, a South American River, etc. There’s extra stuff to do like salvage treasure for money and try to find legendary creatures.
EO2 has more in the way of “traditional” gameplay, I would say.