Rule #1: Don't let yourself fall overboard. The boat can sometimes jerk unexpectedly so when moving about, don't let it take you by surprise.

I usually finish up by telling them that when I want them to do something, I will almost always phrase it as a request. It is unlikely that it will ever become necessary, but if I ever just TELL you to do something, do it immediately and without asking questions. Especially if there is a note of urgency in my voice. I'll be happy to discuss the what and why of it later, but it's possible that it may become necessary to act quickly to keep you and/or the boat safe. Examples might include "DUCK!", "Hand me that!" (while pointing at something), or even "Go below!".