How hard is it to flip a kayak over?

it depends which way you want to flip it! if you are starting the right way up, then it can be very easy to capsize, just lean over! although you normally learn very quickly how far you can lean without capsizing and it will become second nature, of course it does depend on the type of kayak you are in, different designs behave differently. if you are upside and want to get back up, then yes it is easy to do, but with practice, once you know how to roll a kayak it becomes very easy indeed and requires very little strenght, its all in the technique.