Truly get some advance diving class and learn diving (also your buddy too) - you will learn underwater communication much and skills to cooperate , much better than any silly and noisy device.
I have seen that book useful or not decide by yourself ( fish signals ?)
Scuba Diving Hand Signals - Underwater communication for recreational scuba divers ,
Scuba Diving Hand Signals: Underwater Communication Pocket Companion for Recreational Scuba Divers: Lars Behnke: 9781502488480: Amazon.com: Books
Read the following with a calm, friendly, conversational tone. Insert a few smiles, inquisitive looks, and a couple of laughs. That's how it is intended-----
Um.... I agree with learning more skills, better communication, and improving buddy cooperation.
However... I question the relevance of most of your comment. Underwater communication with hand signals or picture books require the attention of both parties. If your buddy is not looking at you, you can't communicate. Humans have about 190deg of vision. With a mask, that drops to maybe 90deg. Frequently, it is very hard to get the attention of another diver. Until you do, even if you both know sign language, you can't communicate.
So, how do you get the attention of your buddy?
1. poke, grab, pull, or other tactile method - this is a fine method. It has it's limitations. You must be close enough to reach out and touch them. Do you really always dive within an arm's length of your buddy? Really, be honest. If you did stay so close, you would be colliding with them frequently and generally be a nuisance. Usually, a good buddy is within a couple of fin kicks distance. That's good, but still annoying if you want to say "hey, look at that fish". Personally, I don't bother with this method unless the communication needed is important, like "Dude, your crack is showing", or "Time to head back".
2. make noise - this is a fine method, depending on the situation. When diving with a group, it is less than optimal, since you become trained most noises are not your buddy, but someone else trying to attract the attention of their buddy. DMs not withstanding, this method works best when diving alone with buddy, rather than a group.
3. motion - this method sucks. With hopes that you might be withing the 90deg field of view of your buddy, wave your hands or arms frantically, hoping they notice. Or, wait for the moment when your buddy is being a good buddy and checking on you. BTW, how often does a good buddy really check? Once a min? Once every 30 sec? Be honest.
So, the device described by the OP is a method to get the attention of your buddy. Nothing more. It is not intended to enhance communication in any way. It is simply a device to quickly and calmly get your buddy to look at you. After which, real communication may proceed.
After all this, I have an idea. Is recorded speech recognizable underwater? Maybe some frequency adjustments would make it more intelligible. Why not have a waterproof device with 5 playback buttons for pre-recorded messages. Wonder what I would record...
"Hey Bob, look over here" - Bob, of course, is my buddy
"Hey Everyone! Look at that" - point while you play it multiple times
"HELP, I've fallen and I can't get up" - just for fun
"Time to turn around and head back" - good for a DM to use with group
"Hey Bob, your crack is showing" - no explanation needed