Legend of the Sea is an ocean adventure: you explore underwater, encounter what lives there, and work through the challenges the depths present.
The setting does a lot for it — visibility is limited, and the things you can't quite see yet are what keeps you moving forward.
An underwater exploration adventure — you move through ocean environments, deal with what lives there, and progress deeper as you gather what you need.
Yes — it's an adventure game rather than a violent one.
Yes — it runs in the browser with nothing to install.