Water Towable Why Did Our Towable Tube Go Underwater?

Why did our towable tube go underwater? - water towable

We bought a tube with 3 people Cabin towable UFO in the academy, has ridden a person in ... We are slowly filled with water and sank ... What went wrong? It was very difficult to return to the boat ... a water leak under the trailer load was ... but does not seem to work so well .... We return to a different kind? Or, what we need to know ... This is our first time with a trailer load.