Pay attention, something to watch out for is near you. This should always be used when someone may not see your shell coming at them such as when leaving the boathouse. Heads up, shell coming out.
A warning to someone about a pending or probable future action that may affect that person or his/her program, project or activity. [D02322] DSMC
Yelled by rowers when they are carrying a shell to warn you they are trying to get someplace and you are in the way. Best response when you hear "heads up" near you is to duck