This message is a warning rather than an error. It means the water calculations and rendering are taking longer than expected for the current water speed values. You may see the water animation stuttering while this message is quickly repeated.

If your sandbox is stuttering, there are a few ways to reduce the number of missed water rendering targets:

• don’t add so much water to the sandbox;
• get a better GPU;
• change the “water speed” (-ws) values at start up.

You can interactively try water speed values that work for your configuration. When you’ve found workable numbers, those values can be used on the command line during sandbox launch.

Right click in the sandbox window and choose Show Water Simulation Control. The dialog box settings can be changed while the sandbox is running. When you have values that work, use these while launching SARndbox.

-ws <water speed> <water max steps>
Sets the relative speed of the water simulation and the maximum
number of simulation steps per frame
Default: 1.0 30

Is documentation of the command line flags available online?

The “offical” flag docs are in the source code or as output from SARndbox -h. But, long-time user/contributor @maphew has posted a copy of a slightly older set of docs in a wiki.

You’re welcome.