One of the first things to do when making a waterline model out of a full hull model is to determine where to make the cut.
I use a mini-table saw for the cut I got at Harbor Freight Tools.
The saw cuts so fast it actually melts the plastic. I use an X-Acto knife to clean up the edge after I am done.
The blade on this saw looks like it could cut off your arm, but probably only a finger. I am trying to avoid finding out for sure what body parts it can remove. A big cut with the table saw does most of the work in only a few seconds. The rest is just trimming by hand. I got the saw for about a hundred dollars a few years ago. I don't use it much, but it is well worth the money for what it can do.