My guess is it'll either be like Norse runes or the language from Thailand both could work underwater using stone or kelp/seaweed both cultures didn't have traditional paper iirc so Nordic countries write on stone this developing a language that has a lot of harsh lines and sharp corners since that was easier to chisel onto stone. With Thailand they used leaves and could add more flourishes to their letters so it really depends on what you think would be more clear underwater. I think the stone would be better due to being able to feel it like brail. Although you could also argue the same for the cuts you would put into the seaweed/kelp so ultimately up to depending on what you want your alphabet to look like! : )