plastic sheets like water

I've found that it's best to score the smooth side for cutting.  This, whether using score and snap, or using a saw of some sort.

For the former, I find a heavy utility knife a better choice than an xacto or scalper-like cutter.

For the latter, using a fine veneer blade installed "backwards" in a variable-speed saw a good choice.  This puts the teeth of the blade into the "sea" surface, so any splintering is on the back (easier to file & sand clean) side.

Truth be told, my tool of choice for the light lens sheets is a Rotozip with a laminate-cutting bit.  This, however can be kind of spendy for some one who is not going to install a lot of plastic laminate counter tops--legacy of a life time in construction for me.

COOL...never thought about that.....been looking and found Plastruct (http://www.plastruct.com) that had this, but it is only 14 by 24......a little short for the Titanic. Thanks for your input. Mike

Try your local hardware or building supply store.  There are a few of those sheets that go in the light fixtures for suspended ceilings that work as water.