The absolute cheapest underwater camera out there is the Crosstour Action CT 9100. This thing costs less than $60!
Waterproof to 40 m / 133 ft with it’s included underwater housing, it has a 170 degree wide angle lens that let’s you capture much of your surroundings.

The Crosstour Action snaps 20MP photos and 4K video resolution, which are both surprising considering the low price point.

The 2 inch screen allows you to effortlessly view your photos and videos, whilst the simple setup makes it easy to use for beginners, a trait enhanced by the anti-shake technology.

This is a very decent waterproof camera for first time underwater photographers and videographers.

It also features Wi Fi integration as well as multiple mounting accessories plus a remote control and wrist strap!