Maybe fill it with sand and/or dirt and/or rocks? This is all experimental because most people do not deliberately try to sink their beach cats. There's probably a law against it -- like littering, or damaging the sea grass. In these parts, getting rid of a boat usually involves a chainsaw.

Yes, we had a boat that sank, but it was a big monohull sailboat that we were traveling and living on at the time. It was raised by inflating bags inside the boat and gradually lifting it with two tow-truck hoists.

I know, now you want to know why it sank. We were tied up to a new dock in the Hudson River. It had replaced an old dock, and somebody had neglected to remove some steel pilings from the old dock. They were below the surface of the water and were invisible because the water was opaque. While we were gone for the day, the old pilings dug a hole in our hull.

The reason I asked the question about beach cats is because somebody mentioned to me recently that some of the new, high-tech hull construction materials do not use foam core and may not have any inherent buoyancy -- and also do not have auxiliary flotation materials in the hulls. I don't know whether this is true or not, but it started me thinking about it because, as I said, nobody wants to try to sink their boat on purpose just to see whether it floats. So we just take it on faith that it will float. And the assurance or guarantee of a dealer or manufacturer is not going to be much comfort to me if my boat is sinking out from under me in the Gulf Stream.

Anyway, my original question, Do you know for sure you boat won't sink, was actually sort of rhetorical, because how CAN you know for sure? Do manufacturers cut a boat in half crosswise and put it in the water to see if the front and aft halves will float independently of each other? Seems like that would be a really good test.

If you are not sure whether your boat will float and you are worried about it, you should put some kind of flotation into your hulls -- maybe the inflatable bags that someone else suggested. Or you could fill it with that styrofoam packing "popcorn" or lots of lobster pot floats or whatever will fit through your inspection hatches. (If you are going to use little, loose things like that, probably better to put big plastic bags into the hulls, put the flotation stuff into the bags and then seal the bags.)