I really want this mod to become a thing so first before I lose sleep over attempting this mod I want to see if someone is wanting to do this mod.
Do you mean a mod that lets you swim anywhere/turns you into a mermaid instead of putting you in shorts/something like that or a whole complete reskin of the game to make it underwater themed, with new names/images for crops, new character models with little fish tails, and fish replacements for all the animals?
There's a mod that gives the player a mermaid tail =) See here: https://www.nexusmods.com/stardewvalley/mods/271/?
Download Winrar or another program that will let you open .rar files. I'm on a Mac, and I use The Unarchiver. Rar files are decently similar to .zip files--they contain compressed files & can contain multiple files, just like .zips. After unpacking the .rar file using whatever program you ended up getting, just load the mod into your Mods folder like you would with an unpacked .zip file and you should be all set!