After swimming in the sea, some amount of water drips from my nose time to time, for several hours. Normally water drips when I bend forward. Is this a medical problem?
Are you sure it's definitely "water" and not just watery mucous? Pool swimming used to do that to me - now I wear a nose clip, and no longer have the problem (plus as a bonus I look super-attractive).
This happens to me as well when I swim in the ocean, but not when in a pool. I attribute it to the roughness of the ocean which means that are always times when water goes into my nose and ends up in my sinuses. Hours later a spurt of it will rush out, and then later another spurt, and so on. It's annoying, but don't worry about, it doesn't mean you have anything wrong with you. It's just that water can enter the sinus cavities, which are convoluted and can trap water.
(There is of course always some risk of infection, so if you later have pain behind your eyes, cheeks, or forehead, or a thick discolored discharge, or headaches, then you should see a physician.)
Yes, it is normal. Not just after swimming open water, although the waves and unpredictability of conditions make it harder to keep water out of your nose than in a pool. It is also completely normal to have water in your ears, for example.