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There does not seem to be a universal set of signals agreed on by all, and some signals/meanings only make sense for certain kinds of paddling and not for others, but here is a basic set that some kayakers use and can serve you well:
A raised vertical paddle or arm usually means to gather around the paddler giving the signal (you decide if you want to use this signal - for river runners this signal often is used to mean the way ahead is clear)
A raised horizontal paddle or raised fist means stop and hold position.
A waving paddle or arm is usually understood to be a distress signal; use this if you need help
Patting the top of your head with one hand means “are you okay?” The same signal is returned if the paddler is okay, otherwise it is assumed there is a problem.