a way to do it so that it is fare is to have it old fashion, having to have a machine pumping air through a tube that has to be on the surface. this will force you to have to place this machine near the shore and it gives you only a certain distance to go. if you wanted to go further out to sea you would need to make two boats, one with the air pump and the other for you, and maybe a third that will have a storage on it. to run the machine you would need a burning material and the better the material the long you can last. If scuba gear is put in the game then there needs to be a reason for putting them in it, so maybe put treasure chests randomly around and maybe some underwater caves.
to make the mask i really dont care how its done.
to make the tubes maybe add a rubber tree. the more tubes in your inventory the further you can go. the tubes will be in your inventory and every block you go you "loose" 1 tube but as you go back you gain it back. the tubes will not interact with anything(it can go through objects) so you dont need to worry if you go into a cave(if they add underwater caves).
to make the machine(this is not the mask this is just the way you get air to the mask):
put stuff on boats:
then to put more then put the double boat boat
P.S. if something doesnt make sense let me know i can change this post.
also the original idea is not mine, i got the idea from someone else and just put some details and some of my own. ideas
Eh. Why not a diving bell? That's certainly more standard of the first low tech diving apparatus we might see...
Could be made out of any number of things, first off being wood or stone, but if you want to go deeper you'd have to use Iron and Glass.
The first you wouldn't be able to see out of, of course, except for below you, and the second you would have glass to see through around you.
Diving bells would be deployed off of a boat, or off of a block anchored above the water. You'd be limited to up and down movement, but it would aid you in making constructions underwater without have to frequently make trips to the surface for air.
Basically you'd have the diving bell in your hand and you could smack the boat to attach it to it, or a block, and... Tada. Diving bell. Right click to get inside of it.
Whoops. Forgot the glass, it's supposed to be in between the iron >_> And the string is there to be used as a tube like thing for the air to travel through, assuming that we will have an air pump attached to a boat. You would only be able to go x amount of distance under water but be able to breath.