Edit (since I wasn't 100% sober last night, I didn't properly read your post):

So, you're looking for the game "Shark" (1988) (aka: "Underwater Treasure Hunt", (1986)) by Errol Williams...

That game was a shareware game, I doubt that you can legally purchase it anymore (other than maybe directly from the author, that is).

However - you can play it here:
https://archive.org/details/msdos_festival_SHARK

There's also a site, where you can download the .zip file. I won't link to it, but I'm sure, you'll be able to find it yourself.

This game apparently was the only game done by Errol Williams.
And since the game is 36 to 38 years old by now and there doesn't seem to be an address available to contact the author...I'd say: save your $5 to $10 shareware fee.

thanks I found there was a vga version too.

There is a shareware game called vga sharks shareware. This game advertises scuba quest from alivesoft by bill dedes, copyright 1996. similiar type of game that has chapters and comes with 2 other games. Was still available from site, but not not same game.
Thanks for help anyways.