Pretty sure you go to the Zora Falls in Link to the past, talk to the big zora and get the zora flippers. Its the blue tunic in other games though.
When I say level up, meaning you progressivly get better at swimming the more you swim, so eventually swimming would become more convient than walking to get to places, but it has to start out pretty crappy and require some work for the player to obtain.
Can you make the script simply disable dashing/sneaking from Mr.Mo's ABS, because if you do it with sneak/dash, you will keep getting errors to update it.. so it would be like this after a while..
Character_swim_dash_swim_dash_dash_dash_dash... etc
We don't want that, it is like pi, it would go on forever. So if you will change it to when in water and swimming that dash/sneak get disabled.
Another bug I found, when using MrMo's ABS, do you make everything have the "Through" effect, because enemies are coming over the water and attacking. As well you should make another thing that checks if a tile is a wall, disallowing the player to jump out of the water... You could do this using the terrain tags once again. So the player should be the only one able to swim, or even go in water, prevent events from doing this. And set up a wall tag, so the player doesn't jump out of the water onto a wall and get stuck.