I have seen this first hand
1. Check to see if the drain plug is installed.
Boats fill up fast.
2. Remove the tie downs from the transom.
I watched as a person backed down a ramp and the water poured over the transom.
3. Make sure you have a rope tied to the boat that is long enough to reach the dock next to the ramp.
Nothing worse than watching your boat float away.
4. Make sure the parking brake is on before you get out of the tow vehicle.
I remember a car and trailor backing down the ramp and disappearing below the surface.
5. Take it slow and have FUN!