This.

If we assume that the shape of a tire/rim/tube is a toroid (fancy name for donut), the volume is 2 X Pi^2 X R X r^2, where R is the radius of the wheel and r is the radius of the tube (half the width). For a 700x50mm wheel, the volume of air is 3.8 liters. A liter of water weighs about 2.2 pounds, so the bike would need to weigh less than 17 pounds to float. Bye bye, bike.

There probably aren't many 700 X 50 wheeled bikes that weigh less than 17 pounds, so you need to be solidly in fat bike territory to have a bike that reliably floats. a 26" x 2.4" tire would float about 11 pounds.

A 27.5" x 3", the tube would displace 8 liters, or 18 pounds. Two tires means 36 pounds of bike. Now we are getting somewhere A fatbike tire (26"x4") would displace about 14 liters, or 62 pounds of bike and gear. Getting closer. 26"x5" would displace 22 liters, or 96 pounds of bike and whatever or whoever is clinging to it.

No other tube is sealed well enough that it will not eventually sink. After the tires, bar tape and empty water bottles are your only salvation.