Love it! You write phenomenally and so far I love it!
Here's my feedback:
-The distinction between Mermaids and Sirens is particularly interesting to me and I love the uniqueness within the world you are building.
-I think you did a lovely job with the GUI, it's not too crowded but it works well with the story and theme.
-You integrated the implementation of pronouns and appearance spectacularly and in a way that did not take away from the story at all.
-The codex entries are particularly amazing. They are short but very interesting reads and make perfect sense.
-I think you also strike a good balance of cost vs. benefit of certain choices. For example, when you can choose to help a person who has fallen into the water, if you do help them, your relationship with Marina goes down because she disagrees with you. However, if you kill them, you become ruthless. I had to genuinely weigh my options! It was brilliant.
Sorry for the word vomit, but I really think you have something special here! As someone who has tried (emphasis on tried) to write an interactive fiction novel like this, I know how hard and time consuming it is. I can't wait to see what else you have in store!