If the cleanout plug blows out it will likely open the rear hatch and the safety switch will kill the engines. As long as you have the plug tied down and it hasn’t launched itself 200’ in the air you simply put it back in and continue on. Some water could be entering the bilge as water splashes around the cleanout tube but the boat won’t sink as the bilge pump can keep up. You can limit the amount of water entering the bilge by making sure the hose clamp on the rubber tube between the cleanout plug and removable fiberglass tub is tight. It’s a PIA to tighten (you have to open the inspection plates in the engine compartment and reach into the bilge) so a lot of them get left loose.
Problems can occur if the cleanout plug becomes partially dislodged and the engine continues to run (along with that loose hose clamp)...You could flood the engine compartment/bilge. It’s rare but has happened. A high water alarm is cheap insurance.
If the scupper valve is the original plastic one it should be replaced ASAP as it’s likely very brittle and the bilge pump won’t keep up with a 2” hole in your boat below the waterline. Here is the best replacement:
Amazon.com : Attwood 66553-3 Boat Self-Bailing Cockpit Barbed Stainless Steel 1 ½-Inch Scupper Valve : Boating Control Cables : Sports & Outdoors
www.amazon.com
Rebuild kits for the cleanout plugs are available for $50 or so each and make it easy to take them in and out.
Also check your oil lines and look for the original zip ties holding them on (which need to be replaced if they haven’t been). If it doesn’t start easily (like two turns of the key when cold) rebuild the carbs. New spark plugs/good oil etc.
Otherwise have fun.