You need to buy a scuba gear from a store. You attach the tank to your mask and equip on before diving. Be careful, the oxygen in the tank is used up when it is equipped.
You can make friends with some... friends. And they may, or may not, return the vehicle to you. I also personally hardly ever use the vehicle. I travel by foot and use Numpad . to return to base, it is faster, and you get to carry heavy things with you in the process. Immersion-breaking, sure, but so is using the remote control.
